Detection of long living neutral hydrated clusters in laboratory simulation of ionospheric D region plasma

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL.,, doi:./ja, Detection of long living neutral hydrated clusters in laboratory simulation of ionospheric D region plasma H. S. S. Sinha,, Koh Ichiro Oyama,, and S. Watanabe Received May ; revised November ; accepted December ; published January. [] The existence of hydrated cluster ions is known through in situ measurements in the D region of the ionosphere and laboratory simulation experiments. A series of experiments were conducted at Sagamihara, Japan with the intention of detecting some of the ions which, although predicted, had eluded detection in laboratory simulation. The other motivation was to look for heavier ions in laboratory simulations in conditions close to those in the D region. With the availability of better ion mass spectrometers, these could supposedly be detected by rocket measurements. Results of these experiments point to a new aspect, namely, the production of a neutral hydrated cluster molecule, which (a) has ionization potential of less than. ev, (b) has lifetimes in excess of min, and (c) is formed within a limited pressure range. As this neutral cluster molecule has a mass number of, most probably it is NO (H O). A number of other important ions, which were detected earlier in laboratory experiments, were also seen in our data. These include NO + (H O) n,no + (H O) n X, NO + (H O) n,h O + (H O) n,h O + (H O) n X, and O + (H O) n series. A few clusters { + (H O + OH), + (NO + NO) and + (NO + HO )} and molecular ions { + (N H + ), + (HO + ) and + (N H + )} were also detected in these experiments. It was also found that, like the earlier experiments, the concentration of most of the hydrated ions showed an oscillatory behavior. The ion formation was observed only within a limited pressure range, which corresponds to the to km altitude range of the ionosphere. Citation: Sinha, H. S. S., K.-I. Oyama, and S. Watanabe (), Detection of long-living neutral hydrated clusters in laboratory simulation of ionospheric D region plasma, J. Geophys. Res. Space Physics,,, doi:./ja.. Introduction [] The study of atmospheric ions is important as they control ionospheric electrical properties, play an important role in aerosol formation, and can be used to measure trace neutral densities. Hydrated cluster ions have been observed in the lower part of the D region of the ionosphere. Starting from the first in situ measurement of ions in D region by Narcisi and Bailey [], a number of laboratory simulations and model studies were conducted to study the process of ion formation [Shahin, ; Fehsenfeld and Ferguson, ; Dunkin et al., ; Fehsenfeld et al., ; Niles et al., ; Fugono, ]. Excellent reviews on this subject are also available [Ferguson, ; Thomas, ; Reid, ; Smith and Adams, ; Viggiano and Arnold, ; Brasseur and Solomon, ]. Such measurements were conducted using a variety of ion mass spectrometers Institute of Space and Astronautical Sciences, JAXA, Sagamihara, Japan. Physical Research Laboratory, Ahmedabad, India. Plasma and Space Science Center, National Cheng Kung University, Tainan, Taiwan. Faculty of Science, Hokkaido University, Hokkaido, Japan. Corresponding author: H. S. S. Sinha, Physical Research Laboratory Ahmedabad, India. (hsinha@prl.res.in). American Geophysical Union. All Rights Reserved. -//JA equipped with an arrangement of differential pumping. These ions are known to be produced in three body chemical reactions. Observations of these ions are scarce for higher mass numbers (> amu) due to which the full chain of their production mechanisms is not understood quantitatively. [] The production of hydrated cluster ions in laboratory conditions, very similar to those present in the D region of the ionosphere, was first attempted by Fugono []. Sinha et al. [] conducted a similar laboratory simulation experiment at Sagamihara, Japan and reported the observations of many new ions having mass numbers up to amu. Some of the ions observed by Sinha et al. [] could not be detected earlier, neither by in situ measurements, by laboratory simulation experiment of Fugono [], nor by other techniques such as corona discharge in the air [Shahin, ] and flowing afterglow experiments [Fehsenfeld et al., ]. In addition to the production and detection of new ions reported by Sinha et al. [], the experiments at Sagamihara were also aimed at studying the temporal development of different ion species under various UV exposure conditions, in a large mass range of to amu. This communication reports mainly the results based on the temporal development of hydrated cluster ions observed under different UV exposures. A brief description of all the hydrated and cluster ions detected, which has been reported earlier by Sinha et al. [], is also given for the sake of completion.

2 . Experimental Setup [] A plasma chamber having a volume of. cm was used to produce nitric oxide plasma with electron densities and electron temperatures in the ranges to cm and to K, respectively, using an intense EUV source developed at the Institute of Space and Astronautical Science, Japan [Oyama et al., ]. After producing the NO + plasma, a controlled amount of water vapor was introduced in to the chamber. The abundance of positive ions produced inside the chamber was measured by a differentially pumped ion quadrupole mass spectrometer (IQMS). The scanning of ion masses from to amu was done at sweep cycles whose duration could be adjusted a priori. Sweep cycle durations of and s were used for observing the temporal development of various ions. A Langmuir probe was used to measure the electron density and electron temperature. These experiments were done at various NO and water vapor pressures. Results of two cases, viz., () NO and water vapor pressure of Torr each and () NO pressure of Torr and water vapor pressure of Torr are discussed here as they are relevant to the main theme being presented here, viz., the detection of neutral hydrated cluster molecules. Figure shows the schematic of the experimental arrangement used for this study. [] The plasma chamber was a cylindrical chamber with a length of cm and a diameter of cm. A rotary pump and a turbo molecular pump, shown on the top portion of Figure, were used for evacuating the chamber to a pressure in the range of to Torr. Thus, traces of constituents of air, such as N and O, will also be present inside the chamber. These gases could act as third body in recombination reactions which might take place inside the chamber. The left side of Figure shows a UV radiation source. Maximum energy of photons emitted by this source is. ev corresponding to the hydrogen Lyman alpha line (. nm). The UV radiation enters the plasma chamber through an MgFl window. At the bottom right side of the chamber, there are two leak valves through which NO and water vapor are brought inside the chamber. In order to control the pressure of the water vapor, a small control system, which controls the temperature of water through a Peltier cooler, is used. By maintaining the water (ice) temperature in the range of C to C, one can control the water vapor pressure inside the chamber. Shown at the bottom left side of Figure is the IQMS with a differential pumping system. A UV mirror is mounted inside the chamber to direct the UV radiation towards the IQMS. A Langmuir probe is used to diagnose the electron density and electron temperature. A large MgFl window is provided at the extreme right side of the chamber to allow the experimenter an interior view. [] The major source of contamination is the water vapor out gassing from the chamber. As the NO gas also makes the chamber dirty, it was ensured before starting the experiment that the chamber was clean. A cleaning process makes use of heating coils placed all around the chamber. The chamber is normally baked at C for about days. By baking the chamber, only H O gets removed, and NO has to be removed by wiping the chamber out with acetone or ethanol.. Observations.. Pressure Dependence [] An experiment was conducted to see how ion concentration changes with increasing NO pressure at a constant water vapor pressure. Figure shows ion spectra obtained after min of turning on the EUV source, which will be referred to only as the source hereafter, at NO pressures of,.,, and Torr. This experiment was done with IQMS scanning time of min. Water vapor pressure was maintained at Torr in all the four cases. It can be seen very clearly that concentration of almost all ions increases with increase of NO pressure up Figure. A schematic of the experimental setup of Sagamihara ion simulation facility.

3 Ion Current (x - A) (d) (c) (b) (a) Sagamihara, Japan March T + min March T + min November T + min November T + min Ion Mass (amu) Figure. Change in ion spectra with increasing NO pressures of (a) Torr, (b). Torr, (c) Torr, and (d) Torr after min of turning on the source. Water vapor pressure was kept fixed at Torr. The scan time of the ion quadrupole mass spectrometer (IQMS) was min. to Torr, and then the concentration starts deceasing. Thus, the highest concentration of most of the ions was observed when NO and water vapor had equal pressure of Torr... General Nature of Ion Mass Spectrum [] It is found in these experiments that there are two types of buildup of ions. One type is where the peak intensities are attained almost in the first few minutes after source activation. The other type is where the buildup is slow and it takes a long time for the peak intensities to fully form. In such cases, the time required for attaining the peak is on the order of a few tens of minutes. In order to know the constitution of different ions, a list of some of the ions detected in the present experiment along with their mass numbers is given in Table. Figure shows the ion spectra after and min of turning on the source. It can be seen that ions like +, +, and + attain very high intensities even in the first and min. These ions belong to the first category. The development of other ions is not as fast. Figure also shows that ions such as +, +, and + can be seen very distinctly even in the first few minutes. [] Figure shows two spectra taken after min of turning on the source. The top panel of Figure, which pertains to pressure ranges where the ion production is the maximum, shows relatively high intensities for ions such as + and +. These ions belong to the second category. The bottom panel, which corresponds to a different pressure range (where the ion production is not as fast as the top panel), shows reduced intensities compared to the top panel but which are in general much higher than those shown in Figure. This is again in conformity with the observation that the ion production is maximum when the NO and water vapor pressures are equal to Torr. [] In addition to the above features, a number of cluster ions belonging to NO + (H O) n,no + (H O) n X, NO + (H O) n, H O + (H O) n,h O + (H O) n X, and O + (H O) n series (where X is a third body such as NO), +, +, and + as well as molecular ions +, +, and + were also detected. As all of these ions have been reported and discussed extensively by Sinha et al. [], they are not discussed here again. It was also found that like the earlier experiments, the concentration of most of the hydrated ions showed an oscillatory behavior... Unusual Ion Buildup [] In one of the experiments conducted at Sagamihara, the ion production was allowed to continue for about an Table. Mass Numbers of Some of NO Hydrates, Proton Hydrates, and Molecular Ions Observed in the Present Series of Experiments NO Hydrates Proton Hydrates Other Proton Hydrates Molecular Ions + NO + H O + H O + + H O + H O NO N + N H + + NO + (H O) + H O + H O + H O + (H O) NO O + NO + + NO + (H O) + H O + (H O) + H O + (H O) O + + HO + NO + (H O) + H O + (H O) + H O + (H O) NO N + N H + + NO + (H O) + H O + (H O) + H O + (H O) + H O + (H O)

4 Figure. Ion spectra taken at min and min after turning on the EUV source at NO and H O pressure of Torr each. Presence of +, +, and + can be seen very clearly in these spectra in addition to other ion peaks. The scan time of the IQMS was min. Figure. Well developed ion spectra seen after min of turning on the EUV source at NO pressure of Torr (top panel) and at NO pressure of Torr (bottom panel). Water vapor pressure in both cases was Torr. The scan time of the IQMS was min. hour, after which the source was deactivated resulting in immediate disappearance of all ions from the spectrum. It was then decided to turn on the source again to see if the temporal development of ion spectra was similar to that observed when the source was activated the first time. To our utter surprise, the ion +, which took a significant time to attain its peak concentration at the start of the experiment, assumed near peak intensity even in the first scan taken after reactivating the source. This was an unexpected observation. These experiments were, therefore, repeated a number of times to check if some of the ions attain very high concentration immediately upon switching on the source after keeping it off for varying periods of time. The results of all these experiments were same every time.

5 [] Figure shows the development of ions + and + in the first such experiment in which the scanning time for the IQMS was kept as s. On the day of this experiment, the IQMS started working only after min of turning on the source as the power supply of IQMS was not turned on at the start of the experiment by an oversight. It can be seen that both + and + acquire a large intensity (around. na) as soon as the source is activated. After about min, the source was deactivated and then turned on and off several times. This behavior of rapidly attaining near peak intensity (around. na) following source reactivation was repeatedly observed after keeping the source deactivated for,,,, and min. [] In the same experiment, the behavior of + was found to be quite different from those of + and +. Figure shows the buildup of + on November. It can be seen from Figure that + does not attain high intensity immediately after the source is activated. After turning on the source for the first time, it takes to min for the peak intensity to build up to levels of around Ion Current (x - A) Ion Current (x - A) November + UV OFF for Minutes (a) (b) UV OFF for November Minutes + Figure. (a) Development of + ions on November. Pressures of both NO and water vapor was Torr. The time for which the source was turned off is shown below arrows in the lower part of figure. The scan time of the IQMS was s. (b) Development of + ions on November. Other details the same as in Figure a. Ion Current (x - A) November + UV OFF for Minutes Figure. Development of + ; other details the same as in Figure.. na. After about min, the source was deactivated for min and again reactivated. To our utter surprise, + acquired very high intensity (. na) immediately after activating the source. Such high intensities were observed again when the source was activated after keeping it off for,,, and min. It was, therefore, decided to repeat the experiment. While repeating the experiment, two things were kept in mind: (a) to increase the scanning time of the IQMS to min to allow sufficient time for the development of various ions and (b) to keep the source off for durations longer than min, which was the maximum time of deactivation for the November experiment. [] Figure shows the development of + over a period of nearly h at NO and water vapor pressures of and Torr, respectively. After the source is activated for the first time, it takes nearly to min for the peak intensity to build up. After about min, the source was deactivated for a period of min. But when the source was turned on (after min off), the buildup of + Ion Current (x - A) December + UV OFF for Minutes Figure. Development of + ions on December. Pressures of NO and water vapor, were and Torr, respectively. The scan time of the IQMS was min.

6 was instantaneous. After keeping the source on for a few minutes, it was turned off and on for periods of,,,, and min. But every time the source was activated, the buildup of + to near its peak intensity was instantaneous. [] Figure shows another experiment wherein pressures of both NO and water vapor were Torr. As shown above, the intensity of all ions attains maximum value at these pressures. As seen from Figure, the intensity of + was so high that the IQMS range was saturated. Although one cannot estimate the time it took to come to the peak intensity, it can be seen clearly here also that the buildup of + is gradual when the source is turned on for the first time. Next, the source was turned off and on in a fashion identical to that done on December. Every time, the buildup of + to near peak intensity was instantaneous, except for the last two peaks, which were lower in magnitude by a factor of ~. [] Figure shows yet another case when pressures of NO and water vapor, were and Torr, respectively. As seen in the earlier three cases, the intensity of + attained near peak values slowly over a period of to min after the source was turned on for the first time. But when the source was turned off for periods of,,,,, and min and turned on again, the peak intensity was attained almost instantaneously. In this case, one can even see a slight increase in peak intensity every time the source was reactivated. [] In all the three experiments which were performed with a scanning time of min, no other ions showed such a gradual buildup towards the peak intensity when the source was turned on or the first time. Also, the intensity of other ions generally decreased every time when the source was reactivated. The intensity of all ions, except +,was extremely small compared to initial levels when the source was turned on after keeping it off for min.. Results and Discussions.. Pressure Dependence [] The processes of ionization and ion loss (due to recombination and collision with other molecules) are taking Ion Current (x - A) March + UV OFF for Minutes Figure. Development of + ions on March. Pressures of both NO and water vapor were Torr, The scan time of the IQMS was min. Ion Current (x - A) March + UV OFF for Minutes Figure. Development of + ions on March. Pressures of NO and water vapor, were and Torr, respectively. The scan time of the IQMS was min. place continuously. Present observations with our finite set of observations show that the ion formation rate is maximum when the pressures of both NO and water vapor are equal to Torr. Also, there is a lower limit of NO pressure ( Torr) below which no ion formation was observed. This is because at such low pressures ion production rate becomes very small. Similarly, there is an upper limit of NO pressure (. ) Torr beyond which the ion formation ceases. This could be because in the presence of higher number of NO molecules the ion neutral collision frequency would be significantly increased resulting in very large momentum transfer to cluster ions which would result in fragmentation of ions. Similar results were reported by Sinha et al. []. Also, these results are in conformity with the in situ measurements which showed the presence of cluster ions in km region of the ionosphere [Viggiano and Arnold, ], which has similar pressure ranges... General Nature of Ion Mass Spectrum [] A very important feature of our spectra is that a large number of ions belonging to series NO + (H O) n,no + (H O) n X, NO + (H O) n, H O + (H O) n, H O + (H O) n X, and O + (H O) n series were detected. In these experiments, cluster ions such as + (H O + OH), + (NO + NO), and + (NO + HO ) were also detected, in addition to molecular ions such as + (N H + ), + (HO + ), and + (N H + ). We found that as in the earlier experiments, the concentration of most of the hydrated ions showed an oscillatory behavior. In view of the fact that the production and loss mechanisms of all these ions were discussed in detail by Sinha et al. [], they are not being discussed here again. The exception is a particularly an unusual ion buildup, which is discussed in the following section. All other features of the spectrum are similar to those reported by Sinha et al. []... Unusual Ion Buildup [] As shown above, some ions like +, +, and + attain very high intensities even within the first to min of turning on the source. The ion +, on the other hand, consistently showed a very different behavior. Figures

7 show that the ion + takes to min to come to peak intensity when the experiment is first started, and its intensity remains near the peak value for up to min. After min, the source was turned off and on for various periods ( to min) comprising four cases, as shown in Figures. In all cases, the intensity of + attained near peak values immediately after reactivating the source. In case of the March experiment (Figure ), + intensity is slightly less when the source was turned on after being off for and min. We do not know the exact cause of these lower values. Our explanation for unusually high + peaks in all the four cases is the following. Most likely, the + ion is NO + (H O). [] The mechanism of production of NO + (H O) could be as follows. The NO gas present in the chamber gets ionized the moment the EUV source is activated, resulting in the formation of NO +. As the phoionization efficiency of NO at Lyman alpha radiation (. nm) is expected to be around % or so, a large number of NO molecules will also remain inside the chamber. The NO + ions can then undergo three body reactions with NO molecules as the third body. These three body reactions can produce hydrated ions up to NO + (H O) as shown below. NO þ þ ðh OÞ n þno NO þ ðh OÞ n þ NO () [] In Equation (), n varies between and. In our experiment, + builds up with time, and it appears that its net growth rate increases with time. This is possible if the production rate of + is very marginally higher than its loss rate. When the EUV source is deactivated, electronic recombination of cluster ions takes place as given below: NO þ ðh OÞ þe NO ðh OÞ þh O () NO þ ðh OÞ þe NO ðh OÞ þh O () [] It appears that neutral cluster molecules NO (H O) is highly stable and has a large total lifetime, which includes radiative lifetime and nonradiative lifetime. In other words, the lifetime of these clusters is very large. If the photon source is activated again, the cluster NO (H O) can get ionized as a whole. This appears to be the reason for the fact that even if we activate the source after long time, the number of cluster ion + remains very large. These experiments also show that NO (H O) must be a quite stable molecule, as it can remain inside the chamber for periods as long as min. As the photon energy of the source is. ev, the NO (H O) cluster molecule should have an ionization potential less than. ev.. Summary and Conclusions [] The above laboratory simulation experiments show that when the pressures of air, NO, and H O inside the chamber mimic those in the D region of the ionosphere, formation of hydrated cluster ions and molecular ions is promoted. In situ measurements in the D region have detected the presence of cluster ions in the altitude region of km [Viggiano and Arnold, ]. Pressures employed in the present experiment were very similar to those in the km region of the ionosphere. For a fixed water vapor pressure of Torr, the cluster formation starts when the NO pressure reaches Torr, peaks when the NO pressure is Torr, and stops when the NO pressure is. Torr or more. [] Various types of ions which were observed in these experiments have already been reported by Sinha et al. []. These ions belong to hydrated cluster ions series {NO + (H O) n, NO + (H O) n X, NO + (H O) n, H O + (H O) n, H O + (H O) n X, and O + (H O) n }, cluster ions series { + (NO + NON ), + (H O + OH), + (NO + NO), and + (NO + HO )}, and molecular ions { + (N H + ), + (HO + ), and + (N H + )}. The names of various ion series are mentioned here for the sake of completeness. For detailed discussion of these ions, the reader is referred to Sinha et al. []. [] The experiments with source deactivation and subsequent reactivation over a range of described time scales showed the generation of neutral cluster molecule NO (H O) having mass number of. This neutral molecule should have a lifetime of at least min and ionization potential of less than. ev. [] Acknowledgments. This work was performed while one of the authors (H.S.S.S.) held a Visiting Professorship at ISAS, JAXA. Authors would like to thank The Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA) for generously funding this project. We would like to thank Y. Tokuyama for his help in setting up the experiment and in taking the observations. Thanks are also due to Satomi Kawaguchi, Junichi Kurihara, and Yoshiko Koizumi for their contribution by way of discussions and computation. References Brasseur, G., and S. Solomon (), Aeronomy of Middle Atmosphere, nd ed, D. Reidel Publishing, Norwell, Mass. Dunkin, D. B., F. C. Fehsenfeld, A. L. 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