Role of Photolysis Frequency in Enhanced Selectivity and Yield for Controlled Bond Breaking in HOD

Abstract

Quantum dynamical calculations on HOD subjected to different combinations of IR and UV pulses have been made to isolate field attributes which maximize selectivity and yield in the photodissociation of the desired O-H/O-D bond. Results from IR/UV pulse combinations which provide very high selectivity and/or yield are analyzed in detail by using population transfer, probability density flow, and flux variations to obtain microdynamic details favoring selectivity and yield. Results indicate that a 2727 cm (-1) 50 fs Gaussian IR pulse in conjunction with a 46,062 cm (-1) 50 fs Gaussian UV pulse with a time lag of approximately 90 fs between the IR and UV pulses gives 79.1% flux in the H-O + D channel and 6.6% flux in the H + O-D channel, whereas a 3706 cm (-1) 50 fs IR pulse in conjunction with a 51 090 cm (-1) UV pulse gives 9.2% flux in the H-O + D channel and 82.1% flux in the H + O-D channel. A 2727 cm (-1) 50 fs IR pulse in conjunction with a 40,062 cm (-1) 50 fs UV pulse provides the greatest selectivity among the sampled field profiles with a flux branching ratio of H-O + D/H + O-D approximately 487.8, and a 3706 cm (-1) 50 fs IR pulse in conjunction with a 45090 cm (-1) 50 fs UV pulse achieves a flux branching ratio of H + O-D/H-O + D approximately 1354.8.

Effect of Vibrational Pre-Excitation on the Dissociation Dynamics of HOD2+

“…This finding can be explained by the difference in masses between deuterium and proton, which results in different bond stretching frequencies when they are allowed to move on the same repulsive potential. Previous investigations − on the photodissociation of HOD on its first excited-state PES gave similar results. It is important to mention that classical trajectory calculations on HOD 2+ gave the value of branching ratio 8.6, which is quite different from quantum results.…”

“…Influence of other initial conditions, like initial rotational and vibrational states of HOD molecule, on dissociation dynamics of HOD 2+ is yet to understand. Previous studies − showed that weak-field unimolecular photodissociation dynamics of HOD strongly depends on the initial vibrational state of HOD molecule. In this article, we would like to answer the question: Does the dissociation dynamics of HOD 2+ depend on the initial vibrational state of HOD molecule as in the case of HOD photodissociation?…”

An Examination of the Expectation Value Profiles for Average Stretch and Momentum in O−H and O−D Bonds of the HOD Molecule To Determine Their Role in Selective Photodissociation

“…The method employed here is identical with that employed in our earlier investigations, − and as in other previous investigations, − ,,,,,,− , we too have considered only the ground and first excited electronic states with bending angle frozen at its equilibrium value. Rotational motion has been neglected as we are studying events on a femtosecond scale.…”

“…Laser-assisted selective control of products from a chemical reaction is a subject of much importance and intense interest. − The local mode character of the O−H and O−D bonds in HOD and availability of accurate potential energy and dipole moment surfaces has made HOD a popular candidate for detailed investigation of selective bond dissociation. − The first excited 1 B 1 electronic state of HOD is purely repulsive with a saddle point barrier separating the H + O−D and H−O + D channels. Excitation to the 1 B 1 surface causes negligible change in the bending angle and use of preexcited O−H/O−D bonds in the ground electronic state with appropriate laser pulses which will deposit HOD in the desired dissociative channel on the repulsive upper surface has been an effective route to selective bond cleavage in HOD. − ,− ,− ,, These features have made it possible to examine selective dissociation of O−H and O−D bonds using only the ground and first excited potential energy surfaces using reasonable UV pulses − ,,,− ,− in considerable detail and the preferential dissociation of O−H and O−D bonds in H + O−D ← H−O−D → H−O + D has served as a popular prototype for selective photodynamic control of chemical reactions.…”

“…We have mapped the H + O−D/H−O + D branching ratio as a function of UV field frequency for different initial states | m , n ⟩ (with m and n quanta of vibrational excitation in the O−H ( m ) and O−D ( n ) modes) and have established UV frequency regimes and initial states which will provide markedly selective dissociation of O−H/O−D bond as desired. − Fully quantal calculations with nonidealized UV pulses which do not assume instantaneous transfer from the ground to excited surface but take into account the field-induced cross-talk between the two surfaces have been shown to provide an effective route to selective , photodynamic control of bond dissociation in H−O−D. It is our purpose in this paper to map the time evolution of the expectation values of stretch and momenta on both the ground and the excited surface under conditions favoring different outcomes to try and analyze the role of stretch/momentum in effecting preferential control.…”