Faraday Discussions. Frontiers in Spectroscopy

Transcription

1 Faraday Discussions Frontiers in Spectroscopy Vol 150

2 PAPER Spectroscopy and astronomy: H 3 + from the laboratory to the Galactic center Takeshi Oka* Faraday Discussions Received 10th May 2011, Accepted 10th May 2011 DOI: /c1fd00092f Since the serendipitous discovery of the Fraunhofer spectrum in the Sun in 1814 which initiated spectroscopy and astrophysics, spectroscopy developed hand in hand with astronomy. I discuss my own work on the infrared spectrum of H 3 + from its discovery in the laboratory in 1980, in interstellar space in 1996, to recent studies in the Galactic center as an example of astronomical spectroscopy. Its spin-off, the spectroscopy of simple molecular ions, is also briefly discussed. 1. Spectroscopy and Astronomy Spectroscopy is a beautiful field with applications to all sciences; astronomy, physics, chemistry and biology. The two variables in spectroscopy, the frequency and the time, are related by the uncertainty principle Dn$Dt ½p. The applications toward astronomy, where we deal with long times of years, is exclusively frequency-domain spectroscopy while applications toward biology, where we deal with short times of femto-seconds, is more time-domain spectroscopy. Either way, spectroscopy is a sharp scalpel cutting deeply into the vastness and mystery of the Universe and life. Clearly it is impossible for any human to be versed over this huge range in time. With apologies to some of you, I limit my discussion to high resolution spectroscopy applied to astronomy. Astronomy is the oldest science and the one that originated our scientific thinking. Surprisingly it is also the youngest science. I know this well since, being affiliated to Departments of Chemistry and Astronomy, I taught both freshman chemistry and astronomy. While the foundation of chemistry has not changed since I was a student several decades ago, the basics of astronomy such as how stars and galaxies form, how the Universe started, what matter composes the Universe, etc., have drastically changed and are mostly still unknown. With the tremendous mysteries far beyond human imagination, astronomy is a science of perennial youth. High resolution spectroscopy unifies astronomy, physics, and chemistry. It is fitting to discuss this at a Faraday Meeting since Michael Faraday, the greatest experimentalist ever, was the genius of unification electricity and magnetism, electricity and chemistry, magnetism and light have all been unified by him. The Father of the unification of astronomy and chemistry is Joseph Fraunhofer, the Bavarian glass engineer who observed the Fraunhofer spectrum in the Sun. 1 This is a classic case of serendipitous discovery. Without the slightest expectation he unified astronomy and chemistry by showing that the Sun is composed of the same atoms that are on the Earth. The understanding of the Fraunhofer spectrum by Kirchoff and Bunsen was the beginning of both spectroscopy and astrophysics. Ever since, spectroscopy has grown hand in hand with astronomy. Over the years astronomers were spectroscopists and spectroscopists were astronomers: William Herschel, William Huggins, Arthur Schuster, Henri Deslandres, Annie Jump Department of Astronomy and Astrophysics and Department of Chemistry, the Enrico Fermi Institute, the University of Chicago, Chicago, Illinois, 60637, USA. t-oka@uchicago.edu This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

3 Cannon, Ejnar Hertzsprung, just to name a few, and of course Gerhard Herzberg and Charlie Townes, the fine tradition which continues to this day. 2. Foundation of molecular spectroscopy Even in dense regions of interstellar space where the density of gas is on the order of 1000 cm 3, collisions occur only once a year. Molecules are in very pure quantum states and extremely accurate spectroscopy is possible. Very weakly allowed transitions, called forbidden transitions and often overlooked in the laboratory, are observable and play important roles as I will show you later in this talk. Studying such transitions requires a most rigorous spectroscopic theory. I am happy that this conference is in Basel since the spectroscopic theory is based on the mathematics of Leonhard Euler, the greatest mathematician, who was born in this city and studied at the University of Basel. Spectroscopic theory is standing on two pillars. The first is the order of magnitude estimates of various interactions. Molecular calculations are possible because two dimensionless quantities the Sommerfeld fine structure constant a ¼ e 2 /Zc 1/137 and the Born Oppenheimer constant k ¼ (m/m) 1/4 1/10 are much smaller than 1. a gives the order of magnitude of the ratio of electron velocity to the light velocity v e /c, and k 4 gives the ratio of molecular rotation velocity to electron velocity V R /v e. Because of the smallness of those constants, we can expand the Hamiltonian in power series of a and k with good convergence and calculate molecular energies with accuracy. Nuclear and particle physicists do not have this luxury since, for the strong interaction, the corresponding constant g 2 /Zc is larger than 1. The orders of magnitude of various terms are shown in Fig This has not been discussed in previous textbooks of spectroscopy. The second pillar is the symmetry of the molecular Hamiltonian with respect to permutation of identical particles and space inversion. Dirac stated that a total Fig. 1 Summary of orders of magnitude of atomic and molecular interaction energies. W elec, W vib, W rot, W vr, and W cent denote electronic, vibrational, rotational, vibration rotation, and centrifugal interactions, respectively. W s-l, W s-s, W QED, and W eqq denote spin orbit, spin spin, quantum electrodynamical, and electric quadrupole interactions, respectively. W j-s, W i-s, and W i-l denote rotation spin, (nuclear spin) (electron spin), and (nuclear spin) electron orbit interactions, respectively, and W i-i and W i-j represent (nuclear spin) (nuclear spin) and (nuclear spin) rotation interactions respectively. From Ref Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

4 wavefunction should be antisymmetric or symmetric with respect to permutation of fermion or boson, respectively, and coined the term Pauli exclusion principle. 3 Molecular spectroscopy is unique in many fields of science in that the symmetry with respect to permutation of nuclei plays the central role and makes the molecular symmetry so rich and beautiful. Those two pillars, the former quantitative but approximate and the latter qualitative but rigorous are the heart of molecular spectroscopy. So much for the introduction, now I wish to discuss my work on H 3+ as an example of astronomical spectroscopy. 3. The beginning I am not a spectroscopist in the sense Gerhard Herzberg was one. Herzberg once told me (In order to understand molecules) you need to do electronic spectroscopy. Although I took this as a sage advice, I did not follow it since understanding molecules is not my motivation to do spectroscopy. I am an applied spectroscopist; spectroscopy is a tool to do interesting science. Rather than studying many molecules, I want to discover something general and fundamental. For this purpose I avoided some popular fields of spectroscopy internal rotation, free radicals, van der Waals dimers, clusters and others. I studied the selection rules for spectroscopy 4 and collisions. 5 I used microwave double resonance which I inherited from my graduate adviser Koichi Shimoda. I worked as an anarchist in spectroscopy showing that all those selection rules like DJ ¼1, 0, DK ¼ 0, parity + 4 etc. and even ortho 4 ortho, para 4 para are by no means sacred but first approximations. If we look at the symmetry more deeply or if we use a sufficient intensity of radiation, no transition is strictly forbidden. On the other hand for collisions, I worked as a lawmaker. It was thought at that time that molecules change rotational levels more or less randomly by collisions but I found some approximate but intricate selection rules. 6 Those selection rules acquired a new dimension when Charlie Townes discovered interstellar ammonia in In the laboratory where billions of collisions rapidly thermalize molecules, the principle of maximum entropy leads to Boltzmann distribution regardless of the selection rules. In space, however, where molecules do not suffer as many collisions during their lifetimes, non-thermal molecular distributions are more a rule than an exception; to understand them, detailed selection rules are indispensible. 7 In general, kinematics rather than thermodynamics governs interstellar chemistry and subtleties of nature appear in their raw form. Fascinated, I decided to spend the rest of my life studying interstellar molecules. My first work was the radio astronomical discovery of cyano-polyacetylenes H(ChC) n CN with n ¼ 2, 3, and 4 using the Herzberg Institute of Astrophysics 46 m Algonquin Telescope. 8 For the first two we used Harry Kroto s novel laboratory results and for the last my simple calculation. 9 They were by far the largest molecules both in molecular weight and number of atoms. As is well-known, Harry pushed this to higher polymeric carbons and discovered C Thus, astronomical spectroscopy led to a major discovery in chemistry. I was more intrigued by how such exotic molecules are produced so abundantly in the hostile environment of the cold and low density space. Two years prior to our discovery, Herbst and Klemperer 11 and Watson 12 had solved this problem by invoking cosmic ray induced ion-neutral reactions which occur efficiently even at low temperature. In this scheme protonated molecular hydrogen, H 3+, plays the central role. Watson was particularly explicit about this saying in the abstract Due to the widespread abundance of H 2, ion molecule reactions with H 2 and H 3+ can be the chief formation process for small interstellar molecules in a large fraction of the interstellar gas. 12 Clearly finding and studying interstellar H 3+ would be the most fundamental thing to do in astronomical spectroscopy. I dropped all my This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

5 experiments and started to search for the infrared spectrum of H 3+ necessary to discover it in interstellar space. 4. The laboratory spectrum of H 3 + H 3+ was discovered exactly 100 years ago by J. J. Thomson, who studied gaseous discharges throughout his life. First he studied negative charges (cathode rays) and discovered the electron, then he studied positive charges (canal rays) and discovered H years later. He was the first to search for a spectrum of H Subsequently many physicists sought its spectrum and several papers were published claiming its discovery. Herzberg looked for the spectrum since a Clearly I was under his influence. 14b I started to build a spectrometer based on a difference frequency system in This device, developed by Alan Pine 15 based on the parametric principle was (and still is) the only widely tunable laser source in the infrared. After reading several papers on hydrogen plasmas, I was convinced that it was possible to find the H 3 + spectrum in absorption. Surprisingly, specialists thought this was impossible. A Nobel laureate (not Herzberg) likened my attempt to killing an elephant by a slingshot. I had an advantage that I had not worked on ion spectroscopy before and was free from such inhibition. After a concentrated effort of 4 years, the spectrum was found in The observed spectrum was unlike any vibration rotation spectra previously observed. It had no obvious symmetry or regularity (Fig. 2) 17 and some people did not approve it even after the analysis by Jim Watson. Now it is all well established and of course people think that it was an obvious thing to do. Fig. 2 Stick diagram of the laboratory spectrum of the n 2 fundamental band of H 3+. Note the atypical spectral pattern without obvious regularity or symmetry because of the strong vibration rotation interactions. The observation of this spectrum in 1980 triggered the avalanche of molecular ion spectroscopy in the 1980s. From Ref Detection of interstellar H 3 + I immediately started to search for interstellar H 3+. In 1979 at the meeting on interstellar molecules at Mont Tremblant celebrating Herzberg s 75th birthday, I had asked Bill Klemperer whether it would be possible to find H 3+ in interstellar space. His answer was negative; his point was that H 3+ was so active that there would not be much steady state concentration. He was right and that was the reason why it took so long to discover it, but this did not discourage me. My first search at the Kitt Peak Observatory however was a miserable failure. 18 I realized that the technology of infrared astronomy was (and still is) far behind that of radio astronomy and much developmental work was needed. Charlie Townes was then pioneering 12 Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

6 astronomical infrared spectroscopy and I asked his advice for a possible collaborator; he mentioned Tom Geballe, his former student at Berkeley then moving to the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea Hawaii. I am forever grateful for this advice. Tom s prescient choice of stars has been the foremost factor for the great development of H 3+ astronomical spectroscopy. Even with Tom s expertise, however, it took many years of groping without success because of the sensitivity of the spectrometer. 19 Then there was the great news that an infrared absorption line of molecular hydrogen was detected at the NASA Infrared Telescope Facility (IRTF). The crucial factor of the success was the recent installment of an array detector. I knew then that the discovery of H 3+ was imminent since the absorption spectrum of H 3+ was expected to be stronger than that of H We applied for the IRTF in 1994 and 1995, but our proposals were rejected three semesters in a row. On the third rejection, the verdict was The Time Allocation Committee continues to have strong reservations about the technical feasibility of this program. Meanwhile, at UKIRT the plan to upgrade its infrared spectrograph with a new array was under way. After it was installed, Tom attempted an observation using the UKIRT Service program, and within an hour detected the first H 3+ signals toward two infrared stars. 20 Because of the small signal to noise ratio (Fig. 3), some astronomers were skeptical about the detection. After 3 months needed for confirmation and improvement, a paper was sent to Nature. 21 According to Tom 20 The smoking gun had been detected and the viability of the ion-molecule model was confirmed.. we experienced our fifteen minutes of fame (for the 15 years of our quest). Fig. 3 The first detection of interstellar H 3+ at the UKIRT toward two young stars that are deeply embedded in their natal molecular clouds. The doublet at mm and mm are the R(1,1) u and R(1,0) transitions of para- and ortho-h 3+, respectively. From Ref Ubiquity of H 3+ and abundance in diffuse clouds Although it took many years to detect the first signal, once detected, H 3+ was found everywhere and our observations have been a succession of surprises to this day. It was very timely that Ben McCall, then 20 years old entered our graduate school and joined my group at this crucial time. H 3+ was not only found in dense (n 10 4 cm 3 ) This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

7 clouds as predicted but also in diffuse (n 10 2 cm 3 ) clouds where the abundant electrons were thought to destroy H 3+. Surprisingly, our systematic observations of dense 22 and diffuse 23 clouds have established that the H 3+ to hydrogen abundance ratio is 10 times higher in diffuse clouds than in dense clouds, 24 and subsequently this led to a conclusion that, the cosmic ray ionization rate is also 10 times higher. 25,26 Very unexpectedly, H 3+ has emerged as a powerful probe to study the diffuse environment. 7. The Galactic center This brings me to the main theme of this talk, the most fruitful period of my astronomical observations, and the most exciting time of my research life observations and analysis of H 3+ in the Central Molecular Zone (CMZ), a region of radius 200 pc at the Galactic center. This period is more or less after I retired from the University of Chicago and was freed from teaching and administration duties. Again I am more enthusiastic to investigate this center of astrophysical activities rather than many clouds individually Surprise I, intense and broad H 3+ spectra The most thrilling aspect of astronomical spectroscopy is that until we observe, we have no idea what awaits us. The year after our discovery of interstellar H 3+, Tom Geballe, a specialist of the Galactic center, pointed the Telescope toward two bright stars in the area and we were caught by a big surprise. 27 The observed signals from H 3+ were 10 times stronger than in any spectrum we had previously observed (Fig. 4) 28 suggesting that the Galactic center may be the treasure house of H 3+. Also in contrast to the previously observed lines which had sharp and simple Gaussian profiles, spectra toward the Galactic center showed very rich and wide velocity profiles indicating that the gas containing H 3+ are moving with a wide range of extremely high velocities. While the Doppler effect is an archenemy for laboratory spectroscopists, it is a great friend for astronomers. Without the Doppler effect astrophysics would be completely barren. The long path from the Galactic center to us, light years, passes through the three spiral arms and many clouds moving with different velocities and results in intricate velocity profiles for each spectral line. Our first problem was how to discriminate the gas in the Galactic center from the foreground gases. Fig. 4 Spectra of the H 3+ doublet toward the Galactic center source GCIRS 3 and the reddened star Cygnus OB 2 No. 12. The latter is among the strongest H 3+ spectrum toward sources in the Galactic disk. This spectrum revealed a high abundance of H 3+ near the Galactic center from Ref Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

8 7.2. Surprise II, Abundant H 3+ in the metastable J ¼ K ¼ 3 level, high temperature This problem was solved in a clean way by our unexpected discovery of H 3+ in the metastable (J, K) ¼ (3,3) level. 29 Around the turn of the century, large telescopes with 8 meter reflectors had become available. The Japanese Subaru Telescope with its IRCS spectrometer is particularly powerful for a survey because of its wide wavelength coverage. Until then our observations had been limited to H 3+ in the ground (1,1) level of para-h 3+ and in the (1,0) level 32.9 K above it of ortho- H 3+, but the wide coverage allowed us to observe other transitions from higher energy levels. Along with Miwa Goto, we were astounded by the strong and broad absorption of the R(3,3) l transition starting from the (3,3) level. The level is 361 K above the lowest (1,1) level (Fig. 5) and its high population is a clear sign of high temperature. It turned out that the metastable H 3+ is unique to the warm environment of the Galactic center. Since then we have used the R(3,3) l absorption as the fingerprint to discriminate gas in the Galactic center from the rest. Fig. 5 Rotational levels of H 3+ in the ground vibrational state. Levels for ortho-h 3+ are in red while those for para-h 3+ are in blue. The levels with broken lines are forbidden by the Pauli exclusion principle. Blue arrows indicate spontaneous emission due to spontaneous breakdown of symmetry. From Ref Surprise III, Scarce H 3+ in the unstable J ¼ K ¼ 2 level, low density In 2003, the 8 meter Telescope at the Gemini South Observatory on Cerro Pachon, Chile, equipped with high resolution (R ) Phoenix Spectrometer became available. Unlike on Mauna Kea Hawaii where the Galactic center is observable only for 3 to 4 h through thick atmosphere, it is high up at the zenith at its peak in the southern hemisphere and allows 8 to 9 h of observations. Spectra toward the brightest infrared star GCS 3-2 are shown in Fig We see the broad R(3,3) l line representing the warm gas in the Galactic center and the sharp CO lines mostly representing cold and dense gas in the foreground spiral arms. Using them we can separate the gas in the Galactic center. A remarkable aspect is that the R(2,2) l absorption is invisible, that is, the (2,2) level which is 210 K below the (3,3) level is scarcely populated. Here is a classic case of huge population inversion. This is due to the (2,2)! (1,1) spontaneous emission with the lifetime of 27 days, caused by the violation of the selection rules. 31 This 27 days set a standard of time. If collisions are much faster, the (2,2) level is populated but if slower it isn t. The critical density for this spontaneous emission is on the order of 200 cm 3. The absence of the R(2,2) l absorption is a clear evidence that the density of the environment is lower than the critical density. Thus we use the R(3,3) l spectrum as a thermometer and the R(2,2) l spectrum as a densitometer. Using a more detailed model This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

9 Fig. 6 From the top, the R(1,1) l, R(3,3) l, and R(2,2) l absorptions of H 3+ and the R(1) line of the v ¼ 2 ) 0 first overtone absorption of CO toward the brightest star GCS 3-2 in the Quintuplet Cluster observed at the Gemini South Observatory. The vertical scaling of the R(3,3) l, and R(2,2) l absorptions, which demonstrate high temperature and low density, respectively, are multiplied by a factor of 2 and that of the CO absorption is divided by 2 for clarity. From Ref. 30. calculation of thermalization, 32 temperature and density of the environment has been determined to be T 250 K and n # 100 cm 3, respectively Surprise IV, Large cloud dimension and high ionization rate The extremely simple chemistry of H 3+ allows us to interpret the observed abundance of H 3+ in terms of the radial length of the cloud L and the ionization rate z. H 3+ is produced by the ionization of H 2 to H 2+ followed by the proton-hop reaction H 2+ +H 2! H+H 3+. Since the latter is more than a million times faster than the former, the production rate of H 3+ is simply given by the production rate of H 2+, that is, zn(h 2 ). H 3+ in the Galactic center is destroyed by dissociative recombination with electrons with the rate kn(e)n(h 3+ ). Equating them and after some manipulation we obtain for the CMZ, 30 zl ¼ 2kN(H 3+ )(n C /n H ) SV R C/H /f(h 2 ). Using the H 3+ dissociative recombination rate constant at 250 K, k ¼ cm 3 s 1, 33 the carbon to hydrogen ratio in the solar vicinity (n C /n H ) SV ¼ , the increase of the carbon to hydrogen ratio from solar vicinity to the GC, R C/H >3, and the molecular hydrogen fraction f(h 2 ) < 1, we obtain a numerical relation zl > ( cm 3 s 1 )N(H 3+ ). 16 Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

10 Therefore the observed H 3+ column densities give the lower limits for zl. For example, the observed H 3+ column density of N(H 3+ ) ¼ cm 2 toward the brightest star GCS 3-2 gives zl > cm s 1. We cannot separate z and L but if we estimate one, we have the other. If we assume z ¼ s 1, two and one orders of magnitude higher than the values for dense and diffuse cloud, respectively, we have L > 35 pc which is a sizable fraction of the radius of the CMZ. We conclude that the warm and diffuse environment discovered by H 3+ has a sizable volume filling factor and very high ionization rate Surprise V, Ubiquity of the newly found gas from the center to 30 pc east If a cloud is long radially, it is likely long also transversely. Indeed our observations of 15 stars distributed from the center (Sgr A*) to 30 pc East showed high column densities of H 3+ toward them without exception (Fig. 7). They also showed the strong R(3,3) l absorption and the weak R(2,2) l absorption indicating high temperature and low density, respectively. For eight of them we observed H 3+ column densities N(H 3+ ) ¼ ( ) cm 2 which gave zl > ( ) 10 5 cm s This established the high ionization rate at the central region of the Galactic center. We have also established the presence of a new category of gas in addition to the previously reported three kinds of gas, namely, (1) dense and cold molecular gas observed by molecular radio emissions, (2) highly ionized hot ( K) gas in the HII regions observed by radio recombination lines, fine structure lines etc., and (3) the ultra-hot ( K) X-ray emitting plasma. Our discovery has drastically changed the concept of gas in the Galactic center. Fig. 7 The R(1,1) l absorption toward 5 stars located from Sgr A* to 30 pc to the East. All 15 stars observed in this region show strong H 3+ absorptions without exception indicating high ionization rate and large transverse dimension of the warm and diffuse gas revealed by the H 3+ spectroscopy. This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

11 7.6. Surprise VI, Ubiquity of the newly found gas from 140 pc West to 85 pc East Two years ago we started to observe wider regions of the CMZ. Since stars there had not been characterized previously, we needed to find good stars ourselves. We need + bright dust-embedded stars which have smooth infrared spectra usable for the H 3 spectroscopy. Most old (late-type) stars are useless since their spectra are too dense with atomic and molecular absorptions in their own photospheres. Using them for spectroscopy is like using lasers with high amplitude noise; we cannot conduct high sensitivity spectroscopy. Starting from a few millions stars automatically recorded by the satellite borne Spitzer telescope, we have so far found 19 suitable stars (Fig. 8). This is just the beginning and we hope to find perhaps 50 stars eventually. So far we have used several of the selected stars for the H 3+ spectroscopy and our labor intensive star hunting has already been amply rewarded. We have observed H 3+ and CO velocity profiles which are wildly different from what we seen in the central region (Fig. 9). 35 While detailed understandings of the velocity profiles are yet to come, there is no question that the newly found warm and diffuse gas exists very widely in the CMZ. Fig newly found bright dust embedded stars qualified for the H 3+ spectroscopy from 140 pc West (a) to 120 pc East (l). The distances are on the assumption of the Galactic center distance of 8 k pc. The central blue box indicates the location of 15 stars from the center to 30 pc East previously observed. 8. Summary and outlook In summary my quest of interstellar H 3+ has gone through three steps: laboratory spectroscopy from 1975 to 1980, the search for interstellar H 3+ from 1981 to 1996, and the study of the Galactic center from 1997 till now. As a funny story goes, each went through three stages. When I was attempting, people said it was impossible. When I found it, people were skeptical. When it was all established and dust is settled, people said it was obvious. The first two discoveries have gone through the full cycles. They are obvious now. My on-going Galactic center work is in the middle. Three years ago my research proposal was rejected by a granting agency. The verdict was The panel was skeptical about some of the ideas in the proposal. For instance, his estimates of the cosmic ray ionization rates in the Galactic Center are unusually high compared to elsewhere in the ISM. So I am going through the second stage. In a way such negative reaction is encouraging since the more negative people are, the more revolutionary is the finding. I am right at the middle of my work on the Galactic center. It will take at least 5 more years to characterize the Central Molecular Zone to my satisfaction. I have another project. Since the cosmic rays are everywhere, H 3+ is abundant in any astronomical object containing molecular hydrogen. So far we have found H 3+ in planetary ionospheres, dense and diffuse clouds in the Galactic disk, the Galactic center, 18 Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

12 Fig. 9 The R(1,1) l absorption of H 3+ observed toward the brightest star GCS 3-2 (top) at 30 pc East, the a star at 140 pc West (middle), and the i star at 68 pc East (bottom) of Sgr A*. They are in the Quintuplet Cluster, near Sgr E, and Sgr B, respectively. Note the entirely different velocity profiles of the spectra. The top spectrum is from Ref. 30 and the other two spectra are from Ref. 35. and a near-by galaxy but there are many varieties of astrophysical objects left to be explored. The biggest problem of infrared absorption spectroscopy is the atmospheric interference. If in the near future, a satellite borne high resolution infrared spectrometer becomes available, the astronomical spectroscopy of H 3+ will have a quantum jump. 9. Spin-offs So far I have talked about my own work, but I had many brilliant students at the University of Chicago. They did all sorts of fundamental spectroscopy but here I mention two which are related to H 3+. Starting from H 2, we can make H 2+,H 3+, and H by adding and subtracting protons and electrons. Likewise starting from CH 4,NH 3, and H 2 O, we can make many cations and anions (Fig. 10). Needless to say, each of these fundamental molecules has deuterated species which are observable because of the extremely high deuterium fractionation in the cold environment. Herzberg s major discoveries were the spectra of free radicals such as CH 3,CH 2 etc. but he also discovered spectra of molecular ions in the visible, most notably CH + and H 2 O +. My students have studied molecular ions one after the other. Crucial to this development was the discovery of the method of velocity modulation by Rich Saykally. The 3 mm infrared spectroscopy has allowed us to study the molecular ions systematically. We can systematize them as iso-electronic molecules with 10, 9, 8, 7, and 6 electrons and iso-protonic molecules with 5, 4, 3, 2, and 1 protons. Those studies enrich chemistry at the most fundamental level. 36 The impact of all this work on astronomy is just arriving. Last year the HIFI instrument on the Herschel Space Observatory came up with the very exciting discoveries of strong rotational lines of three cations OH +,H 2 O +, and CH +. This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

13 Fig. 10 Molecular ions and radicals containing one carbon, nitrogen, and oxygen atom. (CH 5+,CH 4,CH 3 ) (NH 4+,NH 3,NH 2 ) (H 3 O +,H 2 O, OH ) are isoelectronic with 10 electrons and, like their united atom Ne, they all have singlet ground states. (CH 4+,CH 3,CH 2 ) (NH 3+,NH 2,NH ) (H 2 O +, OH) are isoelectronic with 9 electrons and like their united atom F, they all have doublet ground state. (CH 3+,CH 2,CH ) (NH 2+, NH) OH + are isoelectronic with 8 electrons and like their united atom O, they all have triplet ground states (except CH 3+ ). Isoelectronic and isoprotonic species like CH 4 NH 4+,CH 3 NH 3 H 3 O +,CH 3 NH 3+, NH 2 H 2 O, CH 2 NH 2 H 2 O +,CH 2 NH 2+,NH OH, CH NH OH +,CH NH + have similar structure and dynamic properties. The 3 mm infrared spectroscopy allows us to study all of these species systematically. From Ref. 36. Although we have skimmed the easy ones, the richness of Fig. 10 is yet to be explored. Just to give an example, we have observed the spectrum of protonated methane CH 5+ but we could not analyze the spectrum. I did not even know where to start. 37 I think it will take at least a few decades and possibly more to understand the spectrum of this very fundamental molecular ion. If we replace carbon, nitrogen, and oxygen of this table by silicon, phosphorus and sulfur, or germanium, arsine and selenium, we have equally rich tables. The study of them will be useful for plasma diagnostics in semiconductor engineering and industries. Here is a gold mine yet to be explored using the presently available method. Fig. 10 lists only molecules containing one heavy atoms but we can draw equally rich or even richer diagram for molecular ions containing two heavy atoms and more. Molecular ion spectroscopy is a very rich field yet to be explored. Another project related to H 3+ is the study of its highly excited energy levels. I have done the fundamental band but as soon as H 3+ was discovered in Jupiter, information on overtone band became necessary. Students in Chicago have observed many high energy levels moving the front from the infrared to the visible (Fig. 11). 38 The motivation of this work was to check the rigorous ab initio theory. Since Ko1os and Wolniewicz s ab initio calculation reached spectroscopic accuracy in 1975, H 3+ has been the benchmark for the test. By 2005 the truly ab initio calculations using only natural constants has reached spectroscopic accuracy although non-adiabatic corrections and the QED corrections are yet to be made. Now by using action spectroscopy developed by Stephan Schlemmer, the H 3+ spectroscopy has been pushed deep into visible and perhaps into the ultraviolet. 10. Final remark Finally I wish to count out what I think are the fundamental progresses in astronomical spectroscopy in the last few years other than those mentioned above. Laboratory spectroscopy is making great progress in its sensitivity and resolution with newly developed techniques like cavity ring down spectroscopy and action spectroscopy. If the noise immune cavity enhanced optical heterodyne molecular spectroscopy 20 Faraday Discuss., 2011, 150, 9 22 This journal is ª The Royal Society of Chemistry 2011

14 Fig. 11 The fundamental, hot, overtone, and combination bands of H 3+ observed in Chicago. The observed spectral lines up to the energy level of cm 1 well above the barrier to the linearity at 9913 cm 1, shown by the blue broken line. 38 These accurately measured energy levels served as a benchmark for the most rigorous ab initio theory for the last three decades. Now the action spectroscopy has pushed the level deep into visible up to cm 1. (NICE-OHMS) is realized with its full extent it will be the ultimate. The new astronomical observatories like APEX and Herschel, have induced an avalanche of molecular discoveries. The strong far-infrared absorption spectra of OH +,H 2 O +, CH +, and ClH 2+ reported last year will lead us to new understandings of molecular astrophysics. 39 Projects like ALMA, SOFIA, JWST, and others will cause a great number of molecular discoveries in the very near future. Five years ago Michael Mc- Carthy and his group discovered rotational spectra of anions like C 6 H,C 4 H, C 8 H,C 3 N,CN,C 5 N and detected many of them in interstellar space. 40 I regard this as a fundamental discovery. I also regard the discovery of the rotational spectrum of CH + by Takayoshi Amano last year as fundamental. 41 This molecular ion was observed by Douglas and Herzberg 70 years ago in the visible but its rotational spectrum required a long wait arriving just in time for the Herschel excitement. If he finds submillimeter spectral lines of CH 5+, it will be a major step toward understanding of this fundamental molecular ion. On the theoretical side, Michael Morse and John Maier pointed out that it is possible to observe rotational spectra of nonpolar molecules through their magnetic moments. 41 Since the magnetic dipole moment is typically one hundredth (a) of the electric dipole moment, the transitions are weak in the laboratory but as is well known, the weakness is not a drawback in astronomical spectroscopy. Molecular ions such as HChCH +,HChC-ChCH +, CO 2+ will be observed in interstellar space through such rotational transitions. The discovery of interstellar H 2 O + has introduced an intriguing question of spontaneous emission from the lowest para-h 2 O + level 1 01 to ortho-h 2 O + level It is well know that conversion from ortho-h 2 to para-h 2 occurs rapidly in the presence of a paramagnetic catalyst. For H 2 O +, the paramagnetism is right on the molecule and, unlike for H 2 O, the conversion may be competitive with its collisional relaxations. Finally the 100-year quest for the carriers of the Diffuse Interstellar Bands is still in its infancy. 42 Like astronomy, astronomical spectroscopy is a field of perennial youth. Acknowledgements I thank T. R. Geballe, B. J. McCall, and C. P. Morong for critical reading of this paper. The research has been supported by a generation of NSF grants, last one being NSF AST This journal is ª The Royal Society of Chemistry 2011 Faraday Discuss., 2011, 150,

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