Selective photodissociation of O–H and O–D bonds from ground vibrational state of HOD using simple UV pulses

Abstract

Selective cleaving of both O-H and O-D bonds in HOD is achieved using reasonably simple UV pulses to excite the HOD molecule in its ground vibrational state to the repulsive first excited à ͑ 1 B 1 ͒ surface. Detailed theoretical analysis of population transfer and flux in the H+O-D/H-O+D channels reveals an important preparatory role for the cross-talk between the participating levels and a possible role for the beat structure of the population transfer oscillations in facilitating selective dissociation. Excitation using a 50 fs single color 67 169 cm −1 laser pulse achieves a branching ratio H+O-D/H-O+D=5.64 with 82% flux in the H+O-D channel and 15% in the H-O+D channel. A two color 50 fs laser pulse with frequencies of 54 920 and 52 303 cm −1 provides a branching ratio of H-O+D/H+O-D=2.83 and 52% flux in the H-O+D channel and 18% in the H+O-D channel.

Selective control of HOD photodissociation using CW lasers

“…The frequency band width of our Gaussian pulse is ∼ 1000 cm -1 and since the vibrational separation between levels being manipulated is no less than 2642 cm -1 (V 02 -V 01 ) the mechanistic propositions advocated using these pulses are on a relatively sound footing. 33,35 However, it is clearly useful to examine, if same results can be obtained with CW lasers with no uncertainty in the frequencies employed. Also, since the temporal width of the Gaussian envelope requires considerable time lapse before full power of the pulse can come into play it is useful to examine if the faster build up of full power in CW lasers will also lead to new dynamics allowing desired selectivity with less intense pulses.…”

“…The first excited state à ( 1 B 1 ) of HOD being totally repulsive, most approaches to selective control of O-H and O-D dissociation have been based on designing schemes which will transport the molecule to the desired H + O-D/H-O + D region of the repulsive à ( 1 B 1 ) surface using UV frequencies in the first absorption band. Considerable selectivity in dissociation of the O-H bond has been demonstrated even without prior vibrational excitation of the O-H bond, [8][9][10][11][12]20,30,[32][33][34][35] but in most cases, prior excitation of the O-H bond to ensure its deposition in the H + O-D channel on transition to the repulsive à ( 1 B 1 ) surface has been the more favoured route. [8][9][10][12][13][14][15][16][17][18][22][23][24]29 In case of selective dissociation of the O-D bond via transition to the repulsive first excited surface, prior excitation in the O-D stretch has been necessary.…”

“…9,10,12,18,29,30 This precludes any cross-talk between the two surfaces and the large frequency band width of these narrow pulses does not permit analyses based on manipulation of individual vibrational levels. We have pursued new scenarios [33][34][35] which provide selectivity with large yield for dissociation of any of the two (O-H/O-D) bonds using simple Gaussian UV pulses with temporal widths which lead to sufficiently narrow frequency bandwidth to permit mechanistic analysis based on excitation of individual vibrational levels. This approach has revealed new routes to control via manipulation of vibrational states on the ground surface induced by the pulse mediated cross-talk.…”

“…[33][34][35] Furthermore, to exploit the dependence of photodissociation outcome on the initial vibrational state subjected to the photolysis pulse, we have been advocating the use of field optimized initial state (FOIST) scheme [36][37][38] which attempts to distribute the onus for selective control on both the field attributes and the molecular initial state subjected to the chosen photolysis pulse. The field attributes may be chosen for simplicity and chemical insight, and an optimal initial state is generated using FOIST.…”

“…[8][9][10][12][13][14][15][16][17][18][22][23][24]29 In case of selective dissociation of the O-D bond via transition to the repulsive first excited surface, prior excitation in the O-D stretch has been necessary. 10,12,18,20,21,31,[33][34][35] Most of the theoretical approaches to selective control of HOD photodissociation have used δ-function or very narrow Gaussian pulses with more or less instant transition of the molecule from ground to the repulsive excited surface. 9,10,12,18,29,30 This precludes any cross-talk between the two surfaces and the large frequency band width of these narrow pulses does not permit analyses based on manipulation of individual vibrational levels.…”