Direct observation of the (2) 311U state of Cs2 by resonance enhanced two photon ionization spectroscopy in a very cold molecular beam

Transcription

1 Direct observation of the (2) 311U state of Cs2 by resonance enhanced two photon ionization spectroscopy in a very cold molecular beam Bongsoo Kim Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received 19 April 1993; accepted 29 June 1993) A very cold molecular beam of Cs, is generated using a high temperature pulsed nozzle. We observe new absorption bands of CsZ with an origin near 648 nm by resonance enhanced two photon ionization. From the simulation of the rotational band contours, the bands are assigned to the (2) 3II,(l,)-X XCg+(Oi) and (2) 311,(Oz)-X Xcgf(Ol) transitions. The electronic term values and the dissociation energies are determined as T,= f 0.1 cm- and De= At.9 cm- for the 1, state, and T,= =l=O.l cm- and 0,= cm- for the 0: state. These values as well as the vibrational frequency and rotational constant show good agreement with those from the relativistic quantum mechanical calculations. We show that the potential minimum of the (2) 311u state lies lower than those of the C II, and D 2: state. The potential energy curve of the (2) 311u state crosses that of the C II, state on the inner branch. I. INTRODUCTION Alkali metal dimers have complex potential energy curves that are often coupled to one another by strong interactions and thus provide superb systems for the study of the perturbations and photodissociation dynamics of the excited states.iv3 Furthermore, detailed spectroscopic investigation of the alkali metal dimers would be a first step towards deeper understanding of the molecular properties of small alkali metal clusters, which recently attracted much interest.ti Among the alkali dimers Cs, has the greatest number of states in the visible region and has been studied extensively.7-6 Spectroscopic studies of Cs2 have been hampered, however, by the extreme complexity of the absorption spectrum. Many experiments are done in a heat pipe at temperatures of about 300 C, at which many high vibrational and rotational levels are populated. Consequently, the absorption spectrum only shows very broad bands. The spectrum has been much simplified by employing a continuous supersonic jet. However, the internal cooling is still moderate because of limitation to pumping systems. Typical internal temperatures in the continuous jets were To-50 K and T,- 10 K.16 Since Cs, has a very small rotational constant (-0.01 cm- ) and a low vibrational frequency ( -42 cm- ), there still remain many rovibrational levels substantially populated even at this low temperature. Only by applying very high resolution spectroscopic methods, such as Doppler-free fluorescence excitation spectroscopy, * Doppler-free polarization spectroscopy, 1912 and Fourier transform spectroscopy,13 could the spectral lines be assigned unambiguously and useful molecular parameters determined. Even with these high resolution techniques, the assignment of the spectrum becomes very difficult in the presence of a strong perturbation We have recently built a new high temperature pulsed nozzle, which proves to be extremely effective in reducing the internal temperatures of the alkali dimers Using this nozzle, we generate a very cold beam of Cs, and probe the excited states by the combination of multiphoton ionization and the time-of-flight (TOF) mass spectrometer. This optimum combination provides a greatly simplified excitation spectrum as well as highly enhanced detection sensitivity. The analysis of the excitation spectrum has thus become quite straightforward. Although extensive studies have been done on the excited states of the singlet character of the alkali dimers, much less is known about those of triplet character since transitions from the singlet ground state to triplet excited states are usually very weak because of the electron spin selection rule. In order to clarify the perturbations and predissociation dynamics of the alkali dimers, however, the accurate characterization of the triplet states is essential. Since the ground state of the alkali dimers has 2: symmetry, spectroscopic information on the triplet states of ungerade symmetry would be most useful for the study of perturbations in optically prepared states. Since we eliminated the great majority of lines observed in thermal vapors by extensive cooling, the weak singlet-triplet transitions that have been buried under much stronger singletsinglet transitions now become easily detectable in this very cold beam. From the analysis of the polarization spectroscopic study of Cs, around 580 nm, Demtriider and co-workers showed that the D Zz state has an irregular vibrational spacing variation with u, which was ascribed to the effect of perturbations. 5 Kate et al. I2 identified the perturbing state as the bound (2) 311, state from symmetry considerations. From the perturbed energy levels of the D 2: state, they estimated that the vibrational spacings of the (2) 311u state around cm- above the ground state minimum is about 12 cm-. Their suggested potential curve of the (2) 311u state could explain well the observed shift of the vibrational levels of the D 2: state. Since the level shifts were not observed for the low vibrational levels of the D state, it was predicted that the potential minimum of the (2) 311, state is located above that of the D state. In this paper, we report the first direct observation of this (2) 311u state. We observed transitions to the fine J. Chem. Phys. 99 (a), 15 October American Institute of Physics 5677

2 5678 Bongsoo Kim: The 311u state of Cs, structure components of the (2) 311U state. The term values and dissociation energies of the fine structure components are determined accurately. It is found that the origin of the (2) 311U state is located below those of the C II, and D 2+ u states. II. EXPERIMENT Details of the high temperature pulsed nozzle will be reported elsewhere.21 Briefly, Cs2 was generated by expanding Cs atoms with a few atmospheres of Ar or Kr gas through a modified automobile fuel injection valve. By cooling the solenoid part with water the pulsed nozzle could be heated up to 600 C!. In the experiments reported here, the nozzle temperature was maintained at about 350 C. Since the commercial fuel injector (Nippon Denso Co.) produces long jet pulses ( -2 ms) and has a large diameter (0.8 mm) circular nozzle, an efficient pumping system is necessary. A 10 in. diffusion pump backed by a 950 &min rotary pump was used without any baffles for the source chamber. Liquid nitrogen traps were installed in the chambers to prevent the reaction of Cs with the difiusion pump oils and to reduce the back streaming of the pump oils. The pulsed jet of Cs, (the major species are Cs and Cs,) and seeding gas was collimated by a 1.2 mm diameter skimmer (Beam Dynamics) 6 cm from the nozzle. The skimmed molecular beam was then intersected at right angles by the excitation laser 20 cm downstream from the nozzle. A typical pressure was 1 X 10m4 Torr for the source chamber and 4 X lo- Torr for the detection chamber, while the pulsed nozzle was operated at 4 Hz with 2 atm of Ar backing pressure. Csz ions were generated via resonance enhanced two photon ionization (RE2PI). The excitation source was an excimer laser pumped dye laser with a nominal bandwidth of 0.2 cm-. The ions were accelerated by a double electrostatic field to about 3500 ev and traveled through a 70 cm long field free region and detected by a dual microchannel plate detector. The ion signals were amplified by a fast preamplifier, and the mass- TABLE I. Spectral positions of the C(2) Il,(u )-X 2: (u =O) transitions of Cs, (in cm- ). V Doppler-free polarization spectroscopy (Ref. 2) This work selected signals were averaged by a gated boxcar integrator. In order to avoid saturation and higher order multiphoton effects, the laser power was reduced to less than 50 PJ per pulse in a beam diameter of about 5 mm. The frequency of the excitation laser was simultaneously calibrated by obtaining an optogalvanic spectrum with a Ne cell. Ill. RESULTS AND DISCUSSION Figure 1 shows part of the RE2PI spectrum of the C II,-X 2: transitions of Cs2, where Cs, ions were monitored. Cs vapor was expanded with 1000 Torr of Ar. No hot bands are observed in the spectrum. By simulating the rotational contour with known molecular parameters for the X Xi and C II, states,2 the rotational temperature in the beam is calculated to be - 1 K. The vibrational temperature is estimated to be less than 5 K. The peak positions of the C-X progression show very good agreement with those calculated from the high resolution polarization spectroscopic result2 within the accuracy of our measurement ( ho.1 cm- ). Table I compares the peak positions of our spectrum with those estimated from the high resolution study. Figure 2 shows new transitions of Cs2 which start to show up near 648 nm. Cs vapor is expanded with 1000 I I I FREQUENCY/cm FIG. 1. RE2PI spectrum of Cs,, for which the seeding gas was loo0 Torr of Ar. C< ions are monitored. The peaks show the vibrational progression of the C Il,-X 8 transitions. From the simulation of the rotational band contours, we obtain T,- 1 K, and T,< 5 K. J I 1..L I I I FREQUENCY/cm FIG. 2. REZPI spectrum of Csz showing singlet-triplet transitions. Two different series of vibrational progressions are observed and designated as A and B, respectively. The right portion is magnified 40 times. Cs vapor was expanded with 1000 Torr of Ar. The rotational temperature is - 1 K. This spectrum has been constructed by combining two different scans that are linearly scaled. J. Chem. Phys., Vol. 99, No. 8, 15 October 1993

3 Bongsoo Kim: The 311u state of Csp 5679 X60 1, I FREQUENCY/an FIG. 3. REZPI spectrum of Csz, for which the backing pressure is reduced to 100 Torr of Ar. The rotational temperature is increased to -5 K. The left portion is magnified 60 times. The vibrational spacings of the hot bands indicate that the initial state for the transition is the X Z: state. (See Table II.) Torr of Ar. In this spectrum two different series of vibrational progressions are observed (A and B), whose O-O transitions are separated by about 25 cm-. In this cold molecular beam the lowest triplet state, x 3X$, of Csz is also formed in significant concentrations. In order to identify the initial state of the observed transitions, we measured the RE2PI spectrum under different expansion conditions. Figure 3 is the spectrum for the same region obtained at the reduced backing pressure of 100 Torr of Ar. Hot bands are clearly seen in this figure. The Deslandres table for the spectrum in Fig. 3 is shown in Table II. The vibrational spacings of the hot bands clearly show that the initial state of the transitions is the X 2: state. The lowest member of the progression of the 648 nm band is detected at cm-. Since the atomic dissociation limit into [Cs(6 2S1,2> +Cs(6 P3&] lies cm- above the Cs2 X 2; (v=o) state, the electronic state responsible for the 648 nm band should be correlated with the atomic dissociation limit higher than this. By the selection rules for electronic transitions, the final state in the 648 nm transition should be either a lp311 or 32: or 8: state in the presence of strong spin-orb:t coupling. Possible electronic states for which radiative transitions are allowed from the Csz X 8: state are22*23 (i) the (3) 38~(1U,0;) and (4) 38$(1U,0;) states correlated with [Cs( 6 *S1,*) +Cs( 5 20)] and [Cs( 6 2S,,2) +Cs( 7 2S1,2>], respectively, (ii) the (2) II, and (2) X, (O$) states correlated with [Cs( 6 2S1,2) + Cs( 5 20,,2)], (iii) the (2) 311,(2,,1,,Oz,O;) state correlated with [Cs(6 *s,,2) +Cs(5 2&,2)], (iv) the (3) 8: state correlated with [Cs( 6 *S,,,) +Cs(7 2S1,2)], where the terms in parenthesis are for Hund s case (c). Because the singlet states, (2) II,, (2) TABLE II. The Deslandres table constructed from the spectrum in Fig. 3 with the term differences. The energy is given in wave numbers (cm- ) (A) For A progression v \v (41.85 ) (41.69=) (41.53 ) (41.36 ) (B) For B progression v \v *The vibrational spacing of the X X8 state (Ref. 2). J. Chem. Phys., Vol. 99, No. 6, 15 October 1993

4 5680 Bongsoo Kim: The 3Hu state of Cs, X 103cm I I, FREQUENCY/cm- PIG. 4. Comparison of the experimental (top) and calculated (bottom) band contours of the singlet-triplet transitions. For the experimental spectrum Cs vapor was expanded with 100 Torr of Ar. The laser bandwidth was 0.2 cm- and the data points were obtained at intervals of cm -. For the simulated spectrum, r, is assumed 5.0 K and the Q branch is not included for the A band. Same rotational constant for the upper state ( 8 = cm- ) was used to simulate both bands. 8:, and (3) 2: states, are previously identified as the C, D, and E states,15 respectively, the possible candidates for the final state of the 648 nm bands are reduced to the 32,+ and (2) 311U states. While the coupling scheme of angular momenta in Cs, is generally considered as being intermediate between Hund s case (a) and (c), some triplet states may be better described in Hund s case (c).*~ According to the selection rules for rovibronic transitions in Hund s case (c), only P and R branches are allowed for AR=0 transitions, while P, Q, and R branches are allowed for AL? = f 1 transitions. Note that O;- 0: transitions only give a Q branch by the parity selection rule,25 which would be extremely weak. In Fig. 4 the expanded spectrum that shows typical rotational contours for the A and B progressions is presented along with the simulated spectrum. In order to observe richer rotational structures, the rotational temperature of the Cs2 beam is increased by expanding the jet with about 100 Torr of Ar. The difference in the shapes of the rotational contours for the two bands can be ascribed to the missing Q branch for the A progression. Assuming that the rotational temperature is 5 K and B = cm-, we could reproduce the experimental spectrum well by the simulation. The band which belongs to the A progression series is simulated by including only P and R branches and the band which belongs to the B progression series is simulated by including P, Q, and R branches. From these observations and simulations we assign the A progression in Fig. 2 to the 0:-X X+(0+) transition and the B progression to the 1 u -X te8(08) E? 8 transition. : Wnm FIG. 5. The potential energy curves of the C II, D Z:, and (2) II, states. The open squares are from the IPA potential for the C II, state (Ref. 2) and the RKR potential for the D 2: state (Ref. 12). The open circles show the energy of the (2) II, state obtained from the a6 initio calculation of Spiess and Meyer (Ref. 23). The vertical axis is the energy difference from the bottom of the X Ez state, of which the equilibrium internuclear distance (R,) is nrn. Since the (3) 2: state corresponds to the 0; and 1, states in Hund s case (c), the only remaining possibility is the (2) 311, state that is correlated with [Cs( 6 *St,*) + Cs( 5 2D3/2)] atomic dissociation limit. In Hund s case (c), a 311U state corresponds to 2,, l,, O$, and 0; states. Thus we assign the A progression to the (2) 311,(Oz)- X Ed transition and the B progression to the (2) 311,( 1,)-X 22 (0:) transition. The dissociation energies of the 0: and 1 p states [components of the (2) 311, state] were estimated from the electronic term values and the dissociation energy of the X 2: state by using the relation De[(2)311u]= D,(X 2:) +A,?(5 *D si,2)-T,[(2)311,]. For the 0: state, T, = AO.l cm- and D,=2716.7%0.9 cm- and for the 1, state, T,= *0.1 cm- and D,= ho.9 cm-, where the uncertainty is mostly due to that of D,(X El). o, is determined as AO.05 cm- for the 0: state and *0.05 cm- for the 1, state. Recently, more than 50 electronic states of Cs, have been calculated by Spiess and Meye? using the pseudopotential method that includes the relativistic effect. Their calculated term values and the vibrational frequencies for the (2) 311U state are T,= cm- and w,=29.6 cm- for the 0: component and T,= cm- and w,=29.8 cm- for the 1, component. The calculated values for rotational constants are B,=O.Oll 08 cm- for both components. These values show excellent agreement with our experimental measurements. Figure 5 shows the potential energy curve of the (2) 311,(Oz) state along with those for the C II, and D Z$ state. The potential curves for the C II,, and D 2: states are the inverted perturbation approach (IPA) curve and J. Chem. Phys., Vol. 99, No. 8, 15 October 1993

5 Bongsoo Kim: The?iu state of Cs, 5681 the Rydberg-Klein-Rees (RKR) curve derived from the high resolution studies,* * respectively. The potential curve for the (2) 3fI,(O~) state is from the relativistic pseudopotential calculations of Spiess and Meyer.23 Their calculations appear to be very reliable, considering the remarkable agreement of the calculated term values, vibrational frequency, and rotational constant with our measured values. A Morse potential constructed by our measured molecular parameters of the (2) 311u state is similar to the ab initio potential, except for the high vibrational energy region, where the (2) 3H,(O~) state shows an avoided crossing with the D 8: (0, ) state. Note that the potential curves of the C II, ( 1,) and (2) 3TIU states cross at the inner (repulsive) branch. There is a possibility that the (2) 31TU state may contribute to the perturbation of the C ll, state. We should have in mind, however, that in this region other curve crossings occur. Thus, a very complex perturbation is expected. Obviously, the correct description of these electronic states in the crossing region would need more detailed spectroscopic studies. Due to very unfavorable Franck-Condon factors,* the perturbed region (u > 10 of the C II, vibrational levels) cannot be studied in this cold molecular beam. According to Spiess calculation, a repulsive 2: state also crosses both the C II, and (2) 311U states on the outer (attractive) branch. In conclusion, we showed that by employing the new high temperature pulsed nozzle, the absorption spectrum of Cs2 was greatly simplified and the spectral analysis became quite straightforward. The (2) 311U state was directly observed for the first time and the molecular term values were determined accurately. We have established that the origin of the (2) 311U state lies below those of the C ltu and D 2: states. This very cold beam of alkali metals would allow detailed spectroscopic studies of the high-lying electronic states, for which studies have been difficult because of rather limited wavelength coverage of the high resolution lasers. ACKNOWLEDGMENTS The author would like to thank Professor K. Yoshihara for invaluable advice and help during this study. He would like to thank the Inoue Science Foundation and the Japan Society for the Promotion of Science for fellowships. He thanks N. Okada for his help in the construction of the high temperature nozzle and Professor K. Shobatake for the loan of vacuum equipment. Professor H. Kati, and Professor M. Baba are thanked for very helpful advice in the initial stages of the experiments. Discussions with Dr. G. H. Jeung are greatly appreciated. The author would like to thank Professor W. Demtrijder for kindly sending a copy of Dr. N. Spiess thesis. K. H. Meiwes and F. Engelke, Chem. Phys. Lett. 85, 409 (1982). *M Raab, G. Honing, W. Demtroder, and C. R. Vidal, J. Chem. Phys. 76; 4370 (1982). G. Gerber and R. Miiller, Phys. Rev. Lett. 55, 814 ( 1985). 4M. Broyer, G. Delacretaz, P. Labastie, J. P. Wolf, and L. WBste, Phys. Rev. Lett. 57, 1851 (1986). 5. P. Wolf, G. Delacretaz, and L. Waste, Phys. Rev. Lett. 63, 1946 (1989). S. Pollack, C. R. C. Wang, T. A. Dahlseid, and M. M. Kappes, J. Chem. Phys. 96, 4918 (1992). R. Gupta, W. Happer, J. Wagner, and E. Wennmyr, J. Chem. Phys. 68, 199 (1978). sc. B. Collins, J. A. Anderson, D. Popescu, and I. Popescu, J. Chem. Phys. 74, 1053 (1981). 9G. Honing, M. Czajkowski, M. Stock, and W. Demtroder, J. Chem. Phys. 71, 2138 (1979). M. Baba, T. Nakahori, T. Iida, and H. KatB, J. Chem. Phys. 93,4637 (1990). W Weickenmeier, U. Diemer, M. Wahl, M. Raab, and W. Demtroder, J. Chem. Phys. 82, 5354 (1985). *H. Kate, T. Kobayashi, M. Chosa, T. Nakahori, T. Iida, S. Kasahara, and M. Baba, J. Chem. Phys. 94, 2600 ( 1991). C. Amiot, J. Mol. Spectrosc. 107, 28 (1984). 14C. Amiot, J. Chem. Phys. 89, 3993 (1988). C. Amiot, W. Demtrdder, and C. R. Vidal, J. Chem. Phys. 88, 5265 (1988). 16U. Diemer, J. Gress, and W. Demtroder, Chem. Phys. Lett. 178, 330 (1991). B. Kim and K. Yoshihara, Chem. Phys. Lett. 202, 437 (1993). *B. Kim and K. Yoshihara, Chem. Phys. Lett. 204, 407 ( 1993). 19B. Kim and K. Yoshihara, J. Chem. Phys. 98, 5990 ( 1993). * B. Kim and K. Yoshihara, J. Chem. Phys. 99, 1433 (1993). * N. Okada and B. Kim (to be published). 22H. KatB and K. Yoshihara. J. Chem. Phvs. 71, 1585 (1979). 23 N. Spiess, Ph. D. thesis, Fachbereich Chemie, Universitiit Kaiserslautem, C. Amiot and J. Verges, Chem. Phys. Lett. 116, 273 (1985). 25 G. Her&erg, Molecular Spectra and Molecular Structure Z (Van Nostrand, New York, 1950), pp J. Chem. Phys., Vol. 99, No. 6, 15 October 1993