J. Phys. Chem. 1992, 96, 4276-4278
4276
patterns indicate that (CaO),+ clusters prefer cuboid structures and that the most stable structures for the metal-rich clusters are the same cuboids with one 0 atom vacancy for each exmetal atom in the cluster. It appears that the vacancies tend to aggregate on one face of the cluster, eventually leading to a segregated metal layer on a metal oxide cuboid. It may be that the vacancies are occupied by electrons donated from the excess metal atoms and that either these electrons or direct metal-metal bonding stabilizes the vacancy structures.
Acknowledgment. We gratefully acknowledge DuPont Chemicals for an unrestricted grant through the Department of Chemistry, and P.J.Z. thanks them for support through a Pennsylvania State University Particle Science and Technology Center DuPont Fellowship. We also thank Dr. Yasuhiro Yamada and Dr. Andreas Hartmann for helpful discussions during the course of this work and Dr. U. Landman (GIT) for some very helpful suggestions especially regarding the alternative mechanism shown in Figure 5 .
N
Double-Resonance Multiphoton Ionization Spectroscopy of the 6 Rydberg State of Ammonia Teruhiko Nishiya Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: November 12, 1991; In Final Form: January 16, 1992)
Double-resonance multiphoton ionization (MPI) spectra of the B-Rydberg-state of ammonia have been recorded from the symmetric stretching (YJ vibrational level of the ground electronic (X) state (X( 1,O,O,O)level). Symmetry and Franck-Condon (FC) apiderations suggest that the Y,’ vibronic (B(1,0,0,0)) state should be observed from the symmetric (s) inversion component of the X(l,O,O,O) state, but it is shown that only the antisymmetric (a) inversion component actually provides the double-resonance spectra in the wavelength range investigated. Rotational analysis of the bands indicates that an out-of-plane bending (vi) overtone level of the B state (B(0,3,0,0) level) contributes to the spectra. It is suggested that the B(1,0,0,0) state does not have sufficient stability to show double-resonance MPI spectra presumably because of rapid predissociation.
1. Introduction
Ammonia_has a pyramidal equilibrium geometry in its ground electronic (X) state. All of the observed excited electronic states of ammonia are Rydberg in character and have planar equilibrium geometries. A geometry change from pyramidal to planar causes each of the electronic transitions to be dominated by a long progression in the excited-state out-of-plane bending (vi) vibration as a consequence of the Franck-Condon (FC) principle. Therefore, it is short of spectroscopic information about the excited-state stretching vibrations in ammonia, although the stretching vibronic levels may play an important role in the predissociation of the excited states.l Among the first three singkt excited states, the predissociation of the vibronic levels of the A state has most extensively pee; studied by many researchers. Ashfold et al. have observed C-A dispersed emission spectra and have assigned the very diffuse bands to the combination levels of the symmetric strztching (q’) and out-of-plane bending vi) modes of the A state (A(l,n,O,O) levels).* And concerning the ’ state, Miller et al. reinvestigated the 2 + 1 resonance-enhanced multiphoton ionization (REMPI) photoelectron spectra in the vicinity of the b state origin of ammonia cooled in a supersonic jet.3 They reassigned the anomalous photoelectron spectra to the pro ression of the vl’ nu; combination levels of the state (&(l,n,O,O) levels). Related to the work presented here, Seelemann et al. first reported REMPI spectra of vibrationally excited state-selected ammoniae4 They prepared the combination level of the asymmetric stretching (v3) and asymmetric bending ( u 4 ) modes (X(0,0,1,1) level) by near-infrared (near-IR) laser radiation, and performed the 2 + 1 REMPI from that state by ultraviolet (UV)
t:
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(1) Ashfold, M. N. R.; Bennett, C. L.; Stickland, R. J. Comments At. Mol. Phys. 1987, 19, 181. (2) (a) Ashfold, M. N. R.; Bennett, C. L.; Dixon, R. N.; Fielden, P.; Rieley, H.; Stickland, R. J. J . Mol. Speczrosc. 1986, 117, 216. (b) Ashfold, M. N. R.; Bennett, C. L.; Dixon, R. N. Faraday Discuss. Chem. SOC.1986, 82, 163. (c) Dixon, R. N. Chem. Phys. Lett. 1988, 147, 377. (3) Miller, P. J.; Colson, S.D.; Chupka, W. A. Chem. Phys. Lett. 1988, 145, 183. (4) Seelemann, T.; Andresen, P.; Rothe, E. W. Chem. Phys. Lett. 1988, 146, 89.
laser radiation. Allen et al. p e d the method to reveal FC disfavored vibronic levels of the B By rotational analysis of the spectra, they identified the B(O,n,l,O) states. Reported here are_double-resonance multiphoton ionization (MPI) spectra of the B state of ammonia from the X( 1,0,0,0)state. With consideration of the FC principle, chances of det_ectingthe B( 1,O,O,O)state should be much improved using _theX( 1,O,O,O) state as the intermediate state. Preparation of the X(l,O,O,O) state is performed by two excitation schemes. One is an excitation by IR laser radiation. The method is based on the experiment ,Of Seelemann et al.4 and is used in the hope-of observing the B(1,0,0,0) state with the IR excitation of the X( 1,0,0,0) state. And the other is an excitation by stimulated Raman process. Esherick et al. introduced ionization detected stimulated Raman spectroscopy.6 The-method is used with the stimulated Raman pumping of the X(l,O,O,O) state. Comparison of the doubleresonance spectra obtained by both excitation schemes may be able to resolve accidental degeneracies and to ascertain the character of the intermediate state. It is revealed that only the antisymmetric (a) inversion component of the X( 1,0,0,0) state can provide double-resonance MPI spectra in the UV wavelength range studied in !his work. Rotational analysis of the bands indicates that the B(0,3,0,0) state contributes to the spectra. The nonappearance of the B(l,O,O,O) state in this investigation is discussed. 2. Experimental Section
Since the experimental procedure is very similar to that of Seelemann et only the relevant features are described here. Pure ammonia gas is flowed through a glass MPI cell equipped with nickel parallel plates with a bias voltage of 80 V. The cell pressure is measured with a capacitance manometer (MKS, Baratron Type 220B) and is typically 90 mTorr. The ion signals from the cell are preamplified (Keithley, Model 427) and are processed with boxcar averagers (Stanford Research Systems, Model SR25O). ~
(5) Allen, J. M.; Ashfold, M. N . R.; Stickland, R. J.; Western, C. M. Mol. Phys. 1991, 74, 49. (6) Esherick, P.; Owyoung, A. Chem. Phys. Lert. 1983, 103, 235.
0022-3654/92/2096-4276%03.00/0 0 1992 American Chemical Society
The 8 Rydberg State of Ammonia
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4211
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Figure 1. PAS spectrum of a 20-Torr sample of ammenia showing the region of P, Q,and R branch transitions of the vI band (X( 1,0,0,0) state).
Tunable IR radiation is generated via difference frequency mixing of the dye laser (Quantel International, Model TDL-50; LDS765 LDS821 mixed dye, band width = 0.3 cm-I) output and the residual fundamental output (1064 nm; 9394 cm-I) of a Nd:YAG laser (Quantel, Model YG571C-10; pulse width 7 ns). For double-resonance MPI spectroscopy with 1R excitation, the IR laser is tuned with photoacoustic spectroscopy (PAS) in a separate cell equipped with a condenser type microphone (SONY, Model ECM-77s). Typical pulse energy for the IR beam is =250 CJ. And tunable UV radiation is produced using a XeCl excimer laser (Lumonics, Model HE-420-SM-B; pulse width 10 ns) pumped dye laser (Lambda Physik, Model FL3002; PTP dye, band width 0.2 cm-I). Typical pulse energy for the UV beam is =8 mJ. Stimulated Raman pumping of ammonia is accomplished with the m n d harmonic output (532 nm) of the YAG laser and the dye laser (DCM dye) output pumped by the same YAG laser. The same background subtraction scheme as Seelemann et al.4 is adopted using two boxcars (SR250) and an analog processor (SR235). The subtracted signals are transferred via a computer interface (SR245) to a microcomputer (NEC, PC-9801) for subsequent data processing. The background signals of ammonia are also stored for calibration of the UV wavelengths. The UV wavelengths are finally calibrated using the optogalvanic effect in a neon hollow cathode lamp (Hamamatsu, Galvatron Model L2783) and are accurate to fO.O1 nm. Vacuum wavelengths are used in the double-resonance spectra.
UV U S E R WAVELENGTH lnm
Figure 2. Double-resonance MPI spe2tra of a 90-mTorr sample of ammonia with the IR excitation of the X(l,O,O,O) state. The a inversion component of the J = 1 level is prepared via the P branch transition.
+
3. Results and Discussion A. DoubleResonance MPI Spectra. The PAS spectrum of a 20-Torr sample of ammonia is shown in Figure 1. P (AJ = -I), Q (AJ = 0), and R (AJ = +1) branch transitions of the v, band are observed in the spectrum. A high-resolution IR absorption spectrum of the bands near 3 pm and analysis of the stretching bands have been reported.' According to them, the symmetric (s) and antisymmetric (a) inversion components of each J transition in both P and R branches are resolved in Figure 1. K structure is not resolved. Rotational transitions of the nearby u3 and 2 4 bands can accidentally be overlapped into each J transition of the u1 band at the energy resolution of this work. Because the edge region of the dye gain curve is used to generate 3-pm IR radiation (even with the LDS765 LDS821 mixed dye), P branch transitions are weakly observed in comparison with those of the R branch. Figures 2 and 3 show double-resonance MPI spectra of a 90mTorr sample of ammonia with the IR excitation of the X(1,0,0,0) state. I,n Figure 2, the a inversion component of the J = 1 level of the X(l,O,O,O) state is prepared via the P branch transition. The a inversion component of the J = 2 level is prepared via the P branch transition in Figure 3a and via the R branch in 3b, respectively. Up to the J = 6 level, the double-resonance spectra
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Figure 3. Double-resonance MPI spectra of a 90-mTorr sample of ammonia with the IR excitation of the-%( 1,0,0,0) state. The a inversion component of the J = 2 level of the X(l,O,O,O) state is prepared via the (a) P and (b) R branch transitions.
can be obtained. The s inversion component cannot provide doubleresonanceMPI spectra within the range of W wavelengths investigated (335.2-347.2 nm). Seelemann et 51. also carried out state-to-state collision experiments of the X(O,O,1,1) state of ammonia by the same setup as the double-resonance experiments and reported state-to-state collision rate constants to be an order of lo7 Torr-I s-I.~ The product of the sample pressure and delay time between the YAG and excimer lasers of this work is 2 ns Torr. Then, the time constants of collisional processes are 50 times slower than that of detection. Further doubleresonance experiments of a 30-mT01-r sample pressure and IO-ns delay time cause no change in the spectra. The molecular symmetry of ammonia in the rigid molecular frame increases on excitation corresponding to the geometry change from pyramidal (C3")to planar (D3h). However, the appropriate dynamic permutation-inversion group is isomorphic with D3h,which is used herea8 And s and a notation for inversion components indicates that the bending vibrational wavefunction ~~
(7) Benedict, W.S.;Plyler, E. K.; Tidwell, E. D. J. Chem. Phys. 1960, 32, 32.
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(8) (a) Bunker, P. R. Molecular Symmetry and Spectroscopy; Academic: New York, 1979; pp 352-357. (b) Hollas, J. M. High Resolution Specfroscopy; Butterworth: London, 1982;pp 252-256.
4278 The Journal of Physical Chemistry, Vol. 96,No. 1I. I99‘2
is, respectively, symmetric and antisymmetric with respect to inversion. The u1 vibration has A,’ symmetry and v2 has A T in D3,,. Then, the s inversion component of the X(l,O,O,O) state has AI’ symmetry and the a inversion component bas-A,ll. The 2 + 1 REMPI spectroscopy has shown that the B-X transition is carried by the TI2(A)component of the two-photon transition tensor, which transforms as E” in D3h.9,10Thus, the vibronic states of E’ symmetry can be accessiblejia two-photon excitation from the a inversion component of the X(l,O,O,O) state. It is clear from an energetic consideration that the observed double-resonance spectra must result _froma 2 + 1 REMPI process following the preparation of the X( 1,O,O,O)state. Further double-resonance experiments of minus delay time cause np signals. Energetic consideration suggests that the B( 1,0,0,0), B(0,0,1,0), 8(0,0,0,2), and B(0,3,0,0) levels are candidates for the state observed in the double-resonance spectra. Only the B(0,3,0,0) level supports the vibronic state of E‘ symmetry. The spectrum in Figure 2 has the_same rotational pattern as the 2 1 REMPI spectrum of the B(0,3,0,0) state of ammonia cooled in a supersonicjet.” The REMPI spectra show that only the J” = 0, K” = 0 and J” = 1, K” = 0 and 1 levels of the g(O,O,O,O) state are appreciably populated in the supersonic expansion. Since only the a inversion components of the J“ = 1, K” = 1 and J” = 0, K” = 0 levels of the X(0 0 0 0 state has A T symmetry, transitions between those states and the (0,3,0,0) state can be carried by the T12(A) component. In Figure 2, the a inversion component of the J = 1, K = 1 level of t_he X( 1,O,O,O) Gate is populated with the IR excitation. And the X(O,O,O,O) and X(l,O,O,O) states have the same symmetry (Al’). Therefore, the same rotational pattern is observed except the overlapped transition from the J” = 0, K” = 0 level. Rotational analysis of each transition is obtained in Figure 2. The sum of the term value of t . e a inversion component of the J = 1, K = 1 level of the X(l,O,O,O) state (3353.1 C ~ - ~ ) ’and , I ~energy of two UV photons associated with each transition observed in the spectra-coincides with that of the corresponding rotational level of the B(0,3,0,0) state within experimental error ( f l cm-I). Term values of the rotational levels of the B(0,3,0,0) state are derived from highresolution vacuum-UV absorption spectra of ammonia.l23l3 All the transitions satisfy the selection rules (la4 I 2,Iw= 1) of the T12(A)~ o m p o n e n t . ~ , ~ ~ As mentioned before, an accidental overlap of rotational transitions of the v3 and 2v4 bands into each J transition of the u 1 band is possible. Then, comparison of the double-resonance spectra obtained with the P and R branch transitions is carried out in Figure 3. In the case of the preparation of the J = 2 level, transitions observed in Figure 3b coincide with those in Figure 3a. Several transitions are observed only in Figure 3a. This is
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(9) Ashfold, M . N. R.; Dixon, R. N.; Stickland, R. J.; Western, C. M. Chem. Phys. Lett. 1987, 138, 201. (10) Ashfold. M. N. R.; Dixon, R. N.; Little, N.; Stickland, R. J.; Western, C . M.’J. Chem. Phys. 1988, 89, 1754. ( 1 1 ) (a) Conaway, W. E.; Morrison, R. J. S.; Zare, R. N. Chem. Phys. Lett. 1985,113,429. (b) Kay, B. D.; Grimley, A. J. Chem. Phys. Letf. 1986, 127, 303. (12) Urban, S.; D’Cunha, R.; Rao, K. N.; Papousek, D. Can. J . Phys. 1984,62, 1775. (13) Douglas, A. E.; Hollas, J . M. Can. J . Phys. 1961, 39, 479. (14) Chen, K.; Yeung, E. S. J . Chem. Phys. 1978, 69,43.
Nishiya because the selection rules of IR absorption restrict each J transition of the R branch to be from a smaller set of K” levels than that of the P branch. In this case, Figure 3a includes the J = 2, K = 0, 1, and 2 levels and Figure-3b includes the J = 2, K = 0 and 1 levels as the intermediate (X(l,O,O,O)) state of the double-resonance. Since the same transitions are observed in both parts a and b of Figure 3, those transitions are from the same intermediate state. All the transitions satisfy the selection rules of the TI2(A)component. Up to the J = 6 level, the same situation appears. And comparison of the doubleresonancespectra obtained with stimulated Raman pumping and IR excitation is also carried out. Almost all the transitions obtained with stimulated Raman pumping coincide with those obtained with IR excitation. Coincidence between the transitions observed with both excitation schemes further supports that the double-resonance spectra are obtained from the X ( l,O,O,Oi state. B. Nonappearanceof the (1,0,0,0) State in the Double-Resonance Spectra. FC consideration suggests that the 8(l,n,O,O) states (n > 0) are preferred in the double-resonancespectra. And only the range of UV wavelengths corresponding to the 8(1,0,0,0) state is investigated in this work. However, the reported value of the oscillator strengths for the vibronic levels of the 8 state do not s an 1 order of magnitude, at least between the 8(0,3,0,0) and $O,O,O,O) states.Is Therefore, since the 8(0,3,0,0) state is observed in this work, the nonappearance of the 8(1,O,O,O)state in the double-resonance spectra can be attributed to the nature of the B(l,O,O,O) state itself. The results obtained in this work reveal that the 8(1,0,0,0) state does not have sufficient stability to show double-resonance MPI spectra presumably because of rapid predissociation. That is, the signal-to-noise level in this work cannot allow us to detect very diffuse bands due to extensive background subtraction. Therefore, the fact only provides information about the upper limit of the lifetime. Ashfold et al. proposed the predissociation mechanism of the 8 ~ t a t e . ~They J ~ believe that the lifetime_of the 8 state is mainly dominated by a predissociation via the A state with the &A state mixing king a vibroNc mixing involving the u3 vibration. Certainly, as the A state is dissociative along the H2N.-H co2rdinate with only a small barrier, the predissociation of the A(O,n,O,O) states is thought to be a vibrational process, and the vibronic states involving the u3 mode of the A state are expected to be very short lived. And the state (now reassigned as the &O,n,l,O) state1JJ6) has a lifetime about half of that of the B(O,n,O,O) states.” Their mechanism for the predissociation suggests that the fi(l,O,O,O) state has a lifetime of the same order of magnitude as that of the 8(O,n,O,O) states. However, an additional mechanism for the predissociationwould be p i b l e that prefers the 8(1,0,0,0) state. Further investigation is needed to clarify this extra coupling mechanism.
Acknowledgment. I acknowledgethe helpful discussion of Prof. Ichiro Hanazaki and Dr. Masao Takayanagi of the Institute for Molecular Science and also appreciate the technical support of the Equipment Development Center of the Institute. (15) (a) Suto, M.; Lee,L. C. J . Chem. Phys. 1983, 78,4515. (b) Stanley, R. J.; Echt, 0.;Castleman, A. W., Jr. Appl. Phys. 1983, 832, 35. (16) Glownia, J. H.; Riley, S. J.; Colson, S. D.; Nieman, G. C. J . Chem. Phys. 1980, 73,4296. (17) Douglas, A. E. Discuss. Faraday SOC.1963, 35, 158.
