Photodissociation of rovibrationally excited C2H2: Observation of two pathways

Abstract

Articles you may be interested inPhotodissociation of (SO2XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32 040-32090 cm−1 with formation of HSO2 and X Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations C 2 H 2 is prepared in the 2030 0 0 0 ͑five quanta of C-H stretch͒ vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via ͑2ϩ1͒ resonance-enhanced multiphoton ionization ͑REMPI͒ in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a ⌺ u ϩ -⌺ g ϩ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67Ϯ0.10, whereas for R(13) it is 1.34Ϯ0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C 2 H 2 , where the C 2 H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence resonance is involved.

Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

“…Thus, the action intensities of (1410 0 2 0 ) and (2031 1 0 0 ) are close to that of the fourth overtone of the C-H stretch, (2030 0 0 0 ), while their absorption intensities are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated [3][4][5][6][7].…”

“…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.…”

“…Whilst the intensities of the features in the action spectrum depend on both the vibrational excitation probability and the probability that the photolysis of the prepared state produces the tagged photoproduct, the intensities in absorption depend only on the first factor [3,4,5,7,33]. In fact, by determining the ratios between the intensities in the action (I action ) and in absorption (I absorption ) spectra the relative photodissociation cross section for each state can be derived.…”

“…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.…”