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J . Phys. Chem. 1986, 90, 1499-1501
Direct Observation of Photodissociation Pathways in Ammonia by Multiphoton Ionization Photoelectron Spectroscopy Joan B. Pallix and Steven D. Colson* Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 06511 (Received: December 26, 1985)
Excited-state photoelectron spectra from nanosecond and picosecond pulsed excitation_sources have been obtained to directly reveal the relaxation processes_ofoptically pjepared Rydberg states of ammonia. The D(v2=O)state is completely depopulated in 5:1. Therefore, the density of states alone cannot account for the photoelectron intensities observed in Figure 2a. From the photoelectron data alone, one might cogclude that the D levels are actually vibronic components of the C’ state. That this is not the case is known fro_m the energy and from the small, blue deuterium isotope shift of D(u2=O).’ It is not immediately obvious whether the peaks observed for ionization from the B state are due to population-through IC directly from the D sttte or sequentially via the C’ state. In general, IC from the D to the B state will depend upon the multidimensional Franck-Condon factors. Thus, the most efficient energy transfer will occur between states with the minimum difference in the vibrational quantum numbers because all of the states involved have very similar (planar) geo_metries. Therefore, it is most likely that the u2 = 10 leyel of the B state is populated via the low-order overtcnes of the C’ state and not directly from the u2 = 0 level of the D state. A sequential transfer of energy
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1501
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(b B) can then be proposed to explain-the photoelectron peaks observ_ed when optically pumping the D state. That the ratio of the C’(u2=5) to B(u2=_10) peaks is nearly the same for pumping either the D(u2=O) or C’(~2=6)levels (see Figure 2) also supper$ this explanation. B. C’State Photoelectron Spectra. The sp_ectrumshown in Figure 2b is that obtained when pumping the C’(~2=6)level of ammonia directly at 288.0 nm with a 2-11s laser pulse. The most intense peak in this spectrum occurs at 2.00 eV which corresponds to that expected for the Au = 0 ionization transition. The smaller peak in the spectrum occurs at 1.49 eV which corresponds to formation of the ion with 10 u2 quanta. This was not seen in a similar experiment by Glownia et al. because they were using a 3 1 MPI process where the forth photon did not provideznough energy to reach the Au = 0 ionization transition from the B state. From the relative intensities of peaks 1 and ?, it can be concluded that only a fe\?:rovibronic levels from the B state manifold can couple to the C’ state. Then, eve; allowing for rapid equilibration between the coupled B and C’ levels, the resulting phot_oelectronspectrum is predominantly due to ionization from the C’ levels. C. The B State Photoelectron Spectrum. The spectrum shown in Figure 2c is that obtained when pumping the B(u2=10) level of ammonia. The peak l:p_eak 2jntensity ratio also shows the varse coupling between the B and C’ states. The optically pumEd B(u2=10) levels are only coupled to a few of the levels of the C’ manifold at u2 = 5 . The vibronic progression once assigned to the state” of nu2 viammonia has _been reassigned to a progression in u j brations of the B state.’ This vibronic astivity may also be expected in the photoelectron spectrum of the B state of ammonia. Some of the small features on either side of peak 2 can be assigned to members of a progression in u2 of the ground state of the ion. The lowest energy peak (*) is not a member of this progression since it is 0.40 f 0.02 eV ( ~ 3 2 0 0cm-I) from peak 2. However, this interval may be reasonably assigned to u, of the ion since it has a frequency of =34_00 cm-’ in the ground state of ammonia and ~ 2 8 0 cm-’ 0 in the B state. Thjs provides corroborative evidence for the reassignment of the “C state”. D. Conclusions. These studies- conclude-that all internal conversion processes between the D(u2=O), C’(~2=6),and B(u2=10-1 1) vibronic levels occur in less than 5 ps. The optically prep_aredb(u2=0)level decays very rapidly in vibronic levels of the C’ state. This may be the dominanl relaxation patjhway as indicated by the ionization efficiency of C’ levels above D(u2=O). I,f there were another way to rapidly deplete the b state, the? the c‘ ionization efficiency would decrease because of rapid C’ D eqyilibration for C’ levels above D(t2=O), In fact, the C’(~2=6) and C’(u2=7) levels, which bracket D(u2=O), have quite similar ionization efficiencies. There is some evide_ncefor increased line widths associated with C’ levels above the D(c2=O) level in both N H 3 and ND3.I1 However, this is not reflected in a large change in their ionization cross sections. A complete ucderstanding of the very small Au = 0 ionization yield from the D state is left to further _studies, The C’ and B state coupling is less extensive such that their dominant relaxation _(predi_ssociation) pathway must involve processes other than C’ B relaxation. From these observationswe findjhat the D state photochemistry is expected to mimic that of the C’ state and that the C’ and B states should be essentially photochemically independent. Photoproduct analysis following two-photon pumping of these levels is underway to probe the validity of these predictions.
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Acknowledgment. We thank W. A. Chupka for helpful discussions. This work was supportedby the Army Research Office. (11) Nieman,
G.C.;Colson, S . D J . Chem. Phys. 1978, 68,5656.
