Time-dependent Quantum Mechanical Study Research Articles

Photoabsorption of carbon monoxide: a time-dependent quantum mechanical study

Photoabsorption cross-section values are computed for the A1Π–X1Σ+ transition in 12C16O by a time-dependent quantum mechanical method. The computed oscillator strength values are shown to be in good agreement with the available experimental and theoretical values. The small differences between our computed oscillator strength values and the experimental results could be accounted for by the weak interactions between the A1Π state and triplet states (a′3Σ+, e3Σ− and d3Δ) that cross the PEC of the A state near its minimum. Calculations for different isotopologues of CO (12C17O,12C18O and 13C16O) reveal a mass dependence that becomes more significant for higher vibrational levels.

Time-dependent trapping of a molecule

In this paper, we present a theoretical, time-dependent quantum mechanical study of a diatomic, homonuclear molecule subject to an external trapping potential. We choose a Gaussian wave packet to represent the wave function of the center of mass (c.m.) of the molecule, and calculate its propagation numerically using Crank-Nicholson integration. We consider different initial locations of the wave function of the c.m. of the molecule: inside the trapping potential, outside the trap, and partly inside and partly outside. We also examine for each initial location the motion for a range of initial speeds of the c.m. relative to the trap. We calculate the probability that the molecule ends up trapped inside the potential well as well as the probabilities that the molecule will escape the trap by tunneling past or reflecting off the trapping potential. We also reveal the intermediary behavior of the motion, including oscillations in time of the position of the c.m., especially near the edges of the trap, as well as oscillations between bound and unbound molecular states. We compare our results with real systems and experimental work on trapping of molecules at low temperatures.

Low-Energy Electron-Induced Single Strand Breaks in 2′-Deoxycytidine-3′-monophosphate Using the Local Complex Potential Based Time-Dependent Wave Packet Approach

Recent experimental and theoretical investigations on resonant electron scattering off DNA and DNA fragments using low-energy electrons (LEEs), to propose the mechanism for single strand breaks (SSBs) and double strand breaks (DSBs), have received considerable attention. It is our purpose here to understand theoretically the comprehensive route to SSB in a selected DNA fragment, namely, 2'-deoxycytidine-3'-monophosphate (3'-dCMPH), induced by LEE (0-3 eV) scattering using the local complex potential based time-dependent wave packet (LCP-TDWP) approach. To the best of our knowledge, there is no time-dependent quantum mechanical study that has been reported in the literature for this DNA fragment to date. Initial results obtained from our calculation in the gas phase provide a good agreement with experimental observation and show the plausibility of SSB at 0.75 eV, which is very close to the highest SSB yield reported from the experimental measurement (0.8 eV) on plasmid DNA in the condensed phase.

Reaction probabilities and reaction cross sections for three-dimensional He+H2+(v) collisions: A time-dependent quantum mechanical study

Vibrational (v) state-selected reaction probabilities (PvjJ) have been calculated for the reaction He+H2+(v=0,1,2,3)→HeH++H, for H2+ in its ground rotational state (j=0) for a range of total angular momentum (J) values, for total energy (E) in the range 0.94–1.4 eV, using time-dependent quantum mechanical (TDQM) approach and centrifugal sudden approximation. The number of oscillations in Pv0J(E) decreases with increase in J. In addition, there is a noticeable increase in the threshold energy (Eth) with increase in J. The magnitude of Pv0J decreases with increase in J resulting in converged cross section values by the time J becomes 35–45, depending upon v and E under investigation. The resulting (converged) reaction cross section values are in excellent agreement with the experimental results at E=1.14 eV, both in terms of the magnitude and in terms of the observed vibrational enhancement. Interestingly, there is a noticeable discrepancy between our TDQM results and earlier time-independent quantum mechanical studies at higher energies, for v=1 and 2. For v=3, the difference between the two theoretical studies is the largest near the threshold. Our results for v=2 and 3 suggest that the oscillations in the vibrational state-selected reaction cross section values are amenable to experimental observation.

Time-dependent quantum mechanical study of photodissociation of molecular oxygen in the Schumann–Runge continuum

We present a time-dependent quantum mechanical investigation of predissociation of molecular oxygen following absorption in the Schumann–Runge continuum. We calculate the total and partial photoabsorption cross sections in a wave packet formalism in which the predissociation of the B 3Σu− molecular state is attributed to spin-orbit interactions with the Πu1, a 3Πu, Πu5, and 2 3Σu+ repulsive states. We calculate the branching ratio, describing the relative rates of production of excited (1D) and ground-state (3P) oxygen atoms, and find close agreement with a recent measurement.

Isotopic branching in (He, HD+) collisions: A time-dependent quantum mechanical study in three dimensions

A time-dependent quantum mechanical approach has been used to investigate the reaction He+HD+(v=0–4,j=0–3)→HeH++D; HeD++H in three dimensions for total angular momentum J=0. The vib-rotation (v,j) state-selected reaction probability (Pv,jR) is shown to increase with v over the collision energy (Etrans) range (0.95–2.25 eV) investigated for both the exchange channels, in accord with the experimental results. The isotopic branching ratio Γ=PR(HeH+)/PR(HeD+) generally remains less than unity for different v states at different Etrans in agreement with experiment. But at Etrans=1.0 eV, for v=4, Γ obtained from our calculations for j=0 of HD+ is ∼0.8, in excellent agreement with the earlier quasiclassical trajectory calculations, but a factor of 2 less than that obtained from experiment. This difference could arise from the inclusion of nonzero j states in the experimental study, as Pv,jR is found to be j dependent for both the channels. While Pv,jR (HeH+) decreases initially with increase in j from 0 to 2 and then increases when j is increased further to 3, Pv,jR (HeD+) reveals an unusual j dependence; it is larger for even j states of HD+ than for odd j. As a result, Γ is strongly dependent on j, in contrast to the marginal dependence shown by the earlier quasiclassical trajectory calculations.

Photoinduced desorption in NO/Pt: a time-dependent quantum mechanical study

We have carried out a TDQM study of photoinduced desorption in NO/Pt by involving hot electron transfer and allowing different residence times ( τ R) on the excited state potential-energy surface. The desorption probability ( P des) is dependent on τ R, but it is clear from the results that the substantial P des can be accounted for by assuming τ R is in the range 20–60 fs. The study also accounts for vibrational excitation in the desorbed NO.

Time-dependent quantum mechanical study of the photodissociation of HOCl and DOCl

The results of time-dependent quantum mechanical calculations on the photodissociation of initially nonrotating HOCl and DOCl molecules are presented. Two photodissociation processes, 1 1A″←X̃ 1A′ and 2 1A′←X̃ 1A′, are considered. The dynamics are treated in a two dimensional model with a fixed O–H distance and the dissociation process is followed in a rotating body-fixed frame with total angular momentum of J′=1. The total photodissociation spectra and product rotational distributions resulting from dissociation of HOCl and DOCl originating in a number of different vibrational states are presented over a continuous range of incident photon wavelengths in the range 140–440 nm. It is found that, for HOCl, the physically more correct treatment with J′=1 leads to a qualitatively different OH product rotational distribution than that resulting from the more usual model treatment in which both initial and excited states are assumed to possess zero total angular momentum.

Photodissociation and predissociation processes in OH: a time-dependent quantum mechanical study

Photodissociation from the ground vibrational and ground electronic state (X 2Π) and spin—orbit induced predissociation from the A 2∑ + state of OH have been studied using the time-dependent quantum mechanical approach. Photodissociation cross sections computed thus are in accord with the time-independent quantal results and are comparable to the experimental results. Predissociation linewidths (Γ) computed using the wavepacket golden rule treatment corresponding to v′ = 1−7 of A 2∑ + state are also in agreement with most of the earlier theoretical results. For v′ = 1−3, there is a general agreement between experiment and theory but for v′ = 4, the difference is substantial. For v′ = 5−7, there are no experimental results available. The results obtained from our calculations indicate that the inclusion of R-dependence of the spin-orbit coupling plays an important role in deciding the v′ dependence of Γ.

Three-dimensional time-dependent quantum mechanical study of the reaction He + H2 + → HeH+ + H

We report the results of a three-dimensional time-dependent quantum mechanical study of the reaction He + H 2 + (v = 0,1 ) → HeH+ + H at 〈Eintrans 〉 = 1.0 eV, which reproduces clearly the vibrational enhancement for the system. In addition, preliminary results for He + HD+ (v = 1–3) suggest the preferential formation of HeD+ over HeH+ in the products.

Resonances in collinear (He, H +2) collisions. A time-dependent quantum-mechanical study

Results from a time-dependent quantum-mechanical study of the collinear reaction He + H + 2 → HeH + + H are reported in the form of reaction probability values as a function of the total energy of the system for different vibrational states of H + 2. The resonances are related to flux patterns similar to that of trapped trajectories.

Photodissociation dynamics and emission spectroscopy of H2S in its first absorption band: A time dependent quantum mechanical study

A theoretical study of the photodissociation dynamics of H2S in its first absorption band is presented. The potential energy surfaces underlying the dynamics of the breakup process have been modeled so as to reproduce the principal features of all the available experimental data. The modeling is performed using time dependent quantum dynamical methods and involves the exact numerical solution of the time dependent Schrödinger equation. The fitting of the experimental observations requires the use of potential energy surfaces corresponding to two excited electronic states. We have been able to determine two such surfaces which reproduce the observed structure in the absorption spectrum, the main features of the emission spectrum of the dissociating molecule, and the vibrational distribution of the HS photofragments. The calculations utilize a recently developed method for analyzing the wave packet dynamics to extract the partial photodissociation cross sections. The photodissociation process is found to be consistent with an initial excitation to a single excited dissociative diabatic electronic state which is weakly coupled to another, whose principal role within the first absorption band is to introduce diffuse structure into the absorption spectrum and minor perturbations to the nuclear motion. The motion on the principal dissociative electronic surface is dominated by that in the two bond stretching coordinates, while that on the surface of the second perturbing state is a one dimensional vibrational motion which may correspond to either symmetric stretching or bending, but is most probably a complex combination of both. A new method is presented for performing the time dependent quantum mechanical calculation on two coupled potential energy surfaces when the motions on both surfaces are treated in a reduced dimensionality and the dynamically active coordinates on the two surfaces are different.