Predissociation of Excited Acetylene in the ÃA_u State around the Adiabatic Dissociation Threshold as Studied by LIF and H-Atom Action Spectroscopy

Abstract

The LIF and H-atom action spectra of the A ˜-X ˜transition of acetylene in a supersonic jet were observed in the excitation-energy range of 47 000-50 600 cm -1 , where 60 vibrational states were identified in the A state through rotational analyses of respective bands. Most of these states originate from anharmonic couplings of the Franck-Condon-allowed 2 m 3 n states with the 4 i 6 j states where i + j ) even. This fact is consistent with the determined rotational constants of respective states whose values are mostly between those of the 2 m 3 n (m e 1, n e 4) and the 4 1 or 6 1 state. The line intensities of the H-atom action spectra increase at the higher excitation energies, indicating more efficient predissociation. The lifetimes of respective excited states were evaluated from the observed spectral line widths. In a higher excitation-energy region, the lifetime is shortened from 100 ps at 47 000 cm -1 to 25 ps at 50 600 cm -1 , which exceeds the adiabatic dissociation threshold of the A ˜state by 933 cm -1 and is the wavelength limit (197 nm) of the third harmonic of the dye laser. To excite acetylene in the higher excitation energies, vibrationally excited acetylene, which was prepared by irradiation with an IR laser, was excited further by a UV laser. Utilizing this IR-UV double-resonance method, acetylene could be excited up to 53 500 cm -1 , which corresponds to 187 nm. Beyond 50 600 cm -1 , the lifetime of excited acetylene becomes longer, around 100 ps at 51 100 cm -1 . This sudden increase in the lifetime is attributed to the predissociation mechanism changes from the triplet state-mediated dissociation to the direct dissociation on the A ˜state surface. This fact is also supported by the observed average kinetic energies of H atoms estimated from the Doppler line widths of the REMPI spectral lines of H atoms; below 50 600 cm -1 , the kinetic energy is in proportion to the excitation energy, whereas beyond 50 600 cm -1 , the observed average kinetic energies are around 1000-2000 cm -1 , which are a little smaller than the excess energy for the production of C 2 H(A ˜) + H. It was found that the predissociation mechanism or efficiency is not dependent on the vibrational parity, gerade or ungerade, of the initial excited state. This is attributed to the anharmonic coupling between the ν 3 ′ and ν 4 ′/ν 6 ′ vibrational modes.

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

“…These new data have a spectral resolution about 5× better than previous Hatom action spectra of theÃ-X system. 11,36 One particularly important early finding is shown in Figure 1, which shows a hot band going to the K ′ = 2 level of the trans-3 4 6 2 vibrational state. The band is much more prominent in the H-atom action spectrum than in the corresponding LIF spectrum, where overlapping features mostly obscure it.…”

“…4 The transition is now known to be a π * ← π excitation from the linear ground state to a trans-bent excited state, S 1 (à 1 A u ). Further detailed analyses by Watson et al, [5][6][7] the Crim group, 8,9 Yamakita and Tsuchiya, [10][11][12] and Merer et al [13][14][15][16][17] have led to an essentially complete assignment of the vibrational and rotational structure up to 4500 cm −1 above the zero-point level of theà state. One of the most interesting results of these analyses has been the discovery 17,18 of bands that belong to the cis-bent isomer of theà state, the zero-point level of which lies about 2670 cm −1 higher in energy than that of the trans-bent isomer.…”

“…31 As many authors have found, 11,[31][32][33][34][35][36] the diminishing fluorescence quantum yield at higher energies limits the usefulness of laser-induced fluorescence (LIF) for the recording of spectra. Instead, resonance-enhanced multiphoton ionization (REMPI) detection of H atoms has been shown 11,[31][32][33][34][35][36] to be increasingly attractive for detection, since the absorption strength of the transition and the H atom yield both increase above the isomerization barrier. In the present paper, we demonstrate that REMPI spectra at higher resolution permit the observation, with good signal-to-noise, of a predissociated band that is barely observable with LIF detection.…”

Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

“…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.…”

“…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.…”