Photofragmentation dynamics of formaldehyde: CO(v,J) distributions as a function of initial rovibronic state and isotopic substitution

Abstract

Rovibronic state to rovibronic state reaction dynamics: O(3 P)+HCl(v=2,J)→OH(v′,N′)+Cl(2 P) Complete rotational distributions have been obtained for the CO produced following excitation of H 2 CO, HDCO, and D 2 CO near the S 1 origin. The CO was detected by vacuum ultraviolet laser-induced fluorescence. The distributions show a remarkable amount of rotational excitation, peaking at J = 42, 49, and 53 for H 2 CO, HDCO, and D 2 CO, respectively, with widths of 20-25 J units (FWHM). CO(v = 1) from H 2 CO photolysis has nearly the same rotational distribution as CO(v = 0). The population of CO(v = 1) is 14%±5% as large as the population of CO (v = 0), in good agreement with earlier measurements. Increased angular momentum of H CO is only partially transferred to CO, giving slightly wider rotational distributi~ns without changing the peak value. The rotational distributions are highly nonthermal, showing that energy randomization does not occur during the dissociation event. An approximate range of product impact parameters has been determined. The impact parameters are too large to be accounted for by forces along the directions of the C-H bonds. The hydrogen ~ppears to be most strongly repelled by the charge distribution a fraction of an A outside the carbon atom of the CO. The distribution of impact parameters and the internal energy of the hydrogen fragment apparently do not change significantly upon isotopic substitution. The absence of population in CO(J < 20) confirms the identity of CO(J > 25) as the long-lived intermediate in formaldehyde photodissociation.3032

Semiempirical MNDO, AM1, and PM3 direct dynamics trajectory studies of formaldehyde unimolecular dissociation

“…Because of its importance in a number of natural environments, the photochemistry of formaldehyde has been the subject of many experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and theoretical [16][17][18][19][20][21][22][23][24][25][26][27][28][29] studies. The molecular photodissociation of formaldehyde into carbon monoxide and hydrogen is thought to occur on the ground state potential energy surface, after the electronically photoexcited H 2 CO internally converts to highly vibrationally excited electronic ground state H 2 CO. 2 At higher energies, radical dissociation into HϩHCO is also possible.…”

“…3 A number of experiments have yielded the reaction energetics 4,5 and a very detailed description of the product energy partitioning for the H 2 and CO photofragments. 3,[6][7][8][9][10][11][12][13][14][15] Product properties such as translational energy distributions, 3 vibrational and rotational energy distributions, 6-12 quantum state correlations 12,13 and vector correlations [13][14][15] have been reported.…”

Photodissociation Dynamics

“…In the first experiments of this type, rotationally resolved CO distributions were obtained for the photodissociation of CH 2 O and CH 2 CO. 27,28 Since then, CO LIF has been used to study the photodissociation of a series of molecules ranging from CO 2 29 to metal carbonyls. 30,31 Vacuum-ultraviolet LIF has also been used to obtain H 2 (V,J) distributions from the photodissociation of H 2 CO. 32 Virtually all photodissociation/LIF experiments that have been performed subsequent to the examples given here involve the detection of one (or more) of the photofragments mentioned above; the vast number of such experiments obviously precludes mentioning all of them here.…”

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

“…Photodissociation of formaldehyde has been the subject of numerous theoretical and experimental studies (119)(120)(121)(122)(123)(124)(125)(126)(127)(128)(129)(130)(131)(132). It involves two decomposition and one isomerization channels: Downloaded by [George Mason University] at 02: 43 28 September 2014 whereas the isomerization of CH 2 O produces hydroxycarben, HCOH (125,126), the unimolecular decomposition generates H 2 +CO molecular and H • +HC • O radical products.…”