Non-adiabatic dissociation of rovibrationally excited acetylene

Abstract

The photofragmentation dynamics of acetylene, C 2 H 2 , was explored via vibrationally mediated photodissociation. Direct near infrared (NIR) excitation efficiently prepared rovibrational states in the region of three C-H stretch quanta ($9640 cm À1 ), subsequently $243.1 nm UV 1 photons promoted the pre-excited C 2 H 2 molecules to the A ˜1A u state and dissociated them and finally the ensuing H atoms were probed by UV 2 photons via (2 + 1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV 1 dissociating laser wavelength displayed sharp peaks depending on the combined energy and the initially excited rovibrational state. These spectra indicate that the rovibrational transitions belonging to the (1112 0 0 0 ) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030 0 0 0 ) state with three C-H stretch quanta, due to favorable Franck-Condon (FC) factor. The UV absorption from these states sampled particular rovibronic levels of the potential well on the upper A ˜1A u state, disclosing transitions to FC active vibronic modes, involving couplings between torsion and cis-bend with C= =C stretch and trans-bend modes. These results suggest that the initial state preparation samples the bound rather than the purely repulsive region of the A ˜1A u state and that the rovibronic structure is the essential factor in affecting the absorption cross-section. They also indicate that the provided energy is insufficient to overcome the barrier on the A ˜1A u state, leading to the dominant non-adiabatic predissociative photofragmentation into C 2 H (X ~2S + ) + H.

Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

“…The transitions to this dark state are probably characterized by low transition dipole moment for vibrational excitation and by relatively large FC factor for the subsequent UV excitation to the upper state. A similar behaviour was encountered in ethyne and mono-deuterated ethyne, where combination states including bending were preferentially enhanced relative to C-H stretch overtones and consequently showed up preferentially in the action spectra [4,5,9,11,12,14,15].…”

“…C 2 H 2 3.1.1. C 2 H 2 in the 3 CH region [8,11,12] The room temperature PA spectrum in the region of the second overtone, 3 CH stretching of C 2 H 2 , is presented in Figure 2(a). The calculated spectrum shown in Figure 2(b) matches the experiment almost perfectly.…”

“…The VMP of C 2 H 2 was studied for vibrational preexcitation at energies around that of the 1 CH [10] fundamental, and the 3 CH [8,11,12], 4 CH [2,9] and 5 CH [3][4][5][6][7] overtone stretchings. VMP of C 2 H 2 produces H atoms þ C 2 H radicals, while VMP of C 2 HD offers two distinct isotopic channels, H þ C 2 D and D þ C 2 H. To study these channels C 2 HD was excited, alternatively, to three stretches of CH (3 1 ) [7,5], to two stretches of CH plus one of CC and two CCH bends (2 1 þ 2 þ 2 5 ) [5,7], to four stretches of CH and one of CD (4 1 þ 3 ) [6,7], three stretches of CH plus one of Figure 1.…”

“…Before turning to the comparison between the ethyne isotopologues and their larger homologues, we briefly summarize our observations on C 2 H 2 [2][3][4][5][6][7][8][9][10][11][12][13] and C 2 HD [6,7,[13][14][15]. Comparing the action spectra to the rovibrational absorption spectra reveals a wealth of information on the VMP of C 2 H 2 and C 2 HD.…”

“…VMP of the ethyne isotopologues C 2 H 2 [2][3][4][5][6][7][8][9][10][11][12][13] and C 2 HD [6,7,[13][14][15] revealed mode-and bond-selective cleavage of the C-H or C-D bond when the molecules were preexcited to vibrational states in the ground electronic state at energies around that of three, four or five C-H stretches. On the other hand, VMP of three ethyne homologues: (normal) propyne (H 3 CCCH) [16][17][18][19][20], propyne-d 3 (D 3 CCCH) [7,21,22] and 1-butyne (H 3 CH 2 CCCH) [12,19,[23][24][25], pre-excited to energies of up to five C-H acetylenic or alkylic vibrational stretchings, did not show mode-specific cleavage but, nevertheless, in some cases the energy flow out of the pre-excited states indicated mode-dependent coupling with close-lying bath states.…”