Two-photon optically pumped molecular gas visible laser. [Ammonia] Page: 3 of 11
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Gas-phase multiphoton electronic excitation and ionization in amnonia
Las been extensively studied in recent years.1,2 The discovery of several
new electronic states by three-photon absorption (four-photon ionization) has
intensified interest in this simple but important molecule.I Our experiments,
which involve simultaneous two-photon resonant excitation of various vibronic
states, have led to two significant discoveries. We report the first observa-
tion of fluorescence from excited electronic levels of ammonia (the B and C'
states), and, at pressures greater than -200 torr, report the discovery of a
novel high-gain molecular gas laser. Here, simultaneous two-photon absorp-
tion excites a vibronic level of the NH3 C' state from which stimulated
emission (in the visible) occurs down to a vibronic level of the predissocia-
tive A state.
Despite the fact that fluorescence had never previously been observed
from excited states of NH:3, and only weakly from the A state of ND 3, the
expectation of fluorescence from the C' state is based upon lifetime and
selection rule considerations. The united atom selection rules restrict
emission from the C' state to the predissociative A state. Further constraints
arise from dipole selection rules and the Franck-Condon overlap between the
combining vibronic levels, which restrict intense transitions to states of the
same vibrational quantum number. Similar Franck-Condon restrictions apply to
B-+A transitions. The relevant energy levels and photophysical processes arv
shown in Fig. 1. All the excited electronic states of hN3 are predissociated.5
The zero vibrational level of the A state predissociates to yield a ground
electronic state NH2 radical and a hydrogen atom with a quantum yield of
~1.6 Once a transition into the B or C' states has occurred due to intense
laser radiation, the molecule will absorb a third photon and become ionized,
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Quick, C.R. Jr.; Glownia, J.H.; Tiee, J.J. & Archuleta, F.L. Two-photon optically pumped molecular gas visible laser. [Ammonia], article, January 1, 1983; New Mexico. (digital.library.unt.edu/ark:/67531/metadc1110570/m1/3/: accessed January 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.