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

Abstract

The expectation values for stretch and momentum in O-H and O-D bonds of the H-O-D molecule subjected to UV pulses effecting selective cleavage of these bonds have been analyzed to decipher the mechanistic basis of selective dissociation. Fully quantum dynamical calculations using both the ground and excited potential energy surfaces with different initial states and UV pulses reveal that prior stretch in a bond before transferring it to the repulsive first excited-state ensures preferential dissociation of this bond. The sampling of large stretch values and a quick downhill motion in the channel corresponding to dissociation of the prestretched bond on the upper surface are seen to underlie this preferential dissociation.

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.…”

“…These plots indicate that as we move from |0,0⟩ to |1,0⟩ state, counterintuitively, there is an anomalous decrease in flux for the H + + OD + channel and an increase in flux for the D + + OH + channel. This behavior is different from the photodissociation of HOD, where branching ratio for |1,0⟩ state has a larger value than that of |0,0⟩ state . It is also evident from Figure that the flux in H + + OD + channel is larger for |1,0⟩ as compared with that of |0,0⟩ at early time (up to 27.5 fs), but after 27.5 fs, there is a crossing between the flux plots for |1,0⟩ and |0,0⟩ and at long time limit the flux for |0,0⟩ is larger compared with that of the |1,0⟩ case.…”

“…This behavior is different from the photodissociation of HOD, where branching ratio for |1,0⟩ state has a larger value than that of |0,0⟩ state. 17 It is also evident from Figure 5 that the flux in H + + OD + channel is larger for |1,0⟩ as compared with that of |0,0⟩ at early time (up to 27.5 fs), but after 27.5 fs, there is a crossing between the flux plots for |1,0⟩ and |0,0⟩ and at long time limit the flux for |0,0⟩ is larger compared with that of the |1,0⟩ case. To understand this, we looked into the snapshots of the wave packet at different time and found that part of the wave packet for |1,0⟩ state recrosses toward the D + + OH + channel.…”

“…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?…”