Vacuum-ultraviolet stimulated emission from two-photon-excited molecular hydrogen

Abstract

Intense vacuum-ultraviolet stimulated emission in molecular hydrogen, on both the Lyman and Werner bands, following excitation by two-quantum absorption at 193 nm on the X 'Xg~E,F 'Xg transition, has been observed. The shortest wavelength seen in the stimulated-emission spectrum was 117. 6 nm corresponding to the C 'H"~X 'Xs+ (2-5) Q(2) transition. The C 'II"state appears to be populated with a mechanism involving electron collisions. The radiative cascade mechanism found to lead to the vacuum-ultraviolet emissions on the Lyman band also causes strong infrared stimulated emission to occur on the E,F 'Xg~8 'X+"band. Two entirely separate radiative excitation channels are observed to play important roles in the state-selective molecular population of the E,F 'Xg level. One involves two 193-nm quanta in the X~E,F amplitude while the other process combines a 193-nm quantum with a first Stokes-shifted photon in H2. The optical Stark effect was seen to play a significant role in the excitation process with shifts of molecular resonances as large as -45 cm . Substantial deviations from Born-Oppenheimer behavior, resulting in a dramatic shift of the stimulated spectrum depending upon the excited-state rotational quantum number, were clearly observed for molecular levels close to the potential maximum separating the inner and outer wells of the E,F 'Xg state. The maximum energy observed in the strongest stimulated line was -100 pJ, a value corresponding to an energy conversion efficiency of -0.5 Jo. The pulse duration of the stimulated emission is estimated from collisional data to be -10 ps, a figure indicating a maximum converted vacuum-ultraviolet power of -10 MW.

Stimulated VUV Radiation from HD Excited by a Picosecond ArF* Laser

“…As in previous measurements [4,11] in H 2 , the optical Stark effect causes the optimum excitation frequency to be blue-shifted with respect to the unperturbed value, whereas no shift could be detected in the emission spectra, indicating that the stimulated emission occurs slightly later than the excitation. With the unperturbed transition frequencies for the Q(0) and Q(l) transition at 103219 cm" 1 and 103171 cm" 1 , respectively, the actual tv;ophoton frequencies for optimum excitation are ^ 10 3290 cm" 1 and <\i 103200 cm" 1 , respectively.…”

“…As reported previously [4,11] in H2, electron collisions are very efficient in transferring population from the E,F to the C state.…”

“…In a previous experiment, a picosecond ArF* (19 3 nm) laser [3] has been used to two-photon excite the E,F state of mo-. lecular hydrogen [4], The observation of subsequent stimulated emission in the vacuum ultraviolet (VUV) has confirmed the importance of the aforementioned factors. In this paper, it is shown._.…”

“…The most intense emission registered was at 73644 cm" 1 , the B -> X (0 -* 5) p(3) transition. The • excitation and emission pathway is similar to that which applies to H2 [4], Using Frank-Condon factors, which are available for E,F -*• B and B •*• X transitions [9], and Honl-London factors [10], the relative strengths of different rovibrational transitions can be predicted qualitatively and are found to agree with observed line strengths. Naturally, a Quantitative comparison can only be made if the exponential character of a laser process, the effects of saturation, and population depletion are taken into account.…”

Precision measurements and test of molecular theory in highly excited vibrational states of H2 (v = 11)

“…Using Eq. (1), the acStark shift is estimated based on the approximation of Rhodes and coworkers [48,49] for the E-X (2, 0) band. In those studies, they assumed that the intermediate states are predominantly the Rydberg series at principal quantum number n > 2, clustered at an average energy of E m = 14.7 eV.…”