Full-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane

Abstract

We present derivation and implementation of the multiconfigurational strong-field approximation with Gaussian nuclear wave packets (MC-SFA-GWP)-a version of the molecular strong-field approximation which treats all electronic and nuclear degrees of freedom, including their correlations, quantum mechanically. The technique allows realistic simulation of high-order-harmonic emission in polyatomic molecules without invoking reduced-dimensionality models for the nuclear motion or the electronic structure. We use MC-SFA-GWP to model isotope effects in high-order-harmonic-generation (HHG) spectroscopy of methane. The HHG emission in this molecule transiently involves the strongly vibronically coupled 2 F 2 electronic state of the CH 4 + cation. We show that the isotopic HHG ratio in methane contains signatures of (a) field-free vibronic dynamics at the conical intersection (CI); (b) resonant features in the recombination cross sections; (c) laser-driven bound-state dynamics; as well as (d) the well-known short-time Gaussian decay of the emission. We assign the intrinsic vibronic feature (a) to a relatively long-lived ( 4 fs) vibronic wave packet of the singly excited ν 4 (t 2 ) and ν 2 (e) vibrational modes, strongly coupled to the components of the 2 F 2 electronic state. We demonstrate that these physical effects differ in their dependence on the wavelength, intensity, and duration of the driving pulse, allowing them to be disentangled. We thus show that HHG spectroscopy provides a versatile tool for exploring both conical intersections and resonant features in photorecombination matrix elements in the regime not easily accessible with other techniques.

Perturbative and nonperturbative photoionization of H2 and H2O using the molecular R<

“…The enormous complexity of modeling dynamics of continuum molecular states embedded in an external field has led to development of several different ab initio computational approaches, each with their own strengths and weaknesses, thus providing complementary insights into the problem. This includes the haCC approach [6,7], the B-spline algebraic diagrammatic construction (ADC) code [8], the Spanner-Patchkovskii method [9], the multiconfigurational strong-field approximation with Gaussian nuclear wave packets approach (MC-SFA-GWP) [10] capable of describing coupled electronic-nuclear dynamics, the recent extension of the Xchem package [11] to calculations of field ionization of small molecules, the time-dependent multiconfiguration self-consistent-field method based on occupation-restricted multiple-active-space model (TD-ORMAS) [12], the multiconfiguration time-dependent Hartree (MCTDH) method [13,14], time-dependent coupled-cluster (TDCC) method [15][16][17], the TD close-coupling method [18], and others [19].…”

RMT: R-matrix with time-dependence. Solving the semi-relativistic, time-dependent Schrödinger equation for general, multielectron atoms and molecules in intense, ultrashort, arbitrarily polarized laser pulses

“…In larger molecules (e.g. CH 4 ) interacting with strong fields, the coupled electronic-nuclear problem can be solved using the multiconfigurational strong-field approximation with Gaussian nuclear wave packets method [39]. Extending the use of the RMT method to molecules was a natural step to enable calculations of a similar high quality to those that can be performed for atoms.…”

Attosecond Molecular Dynamics and Spectroscopy

“…Varandas, 2015, 2014;Patchkovskii, 2009), and, more recently, in Ref. (Patchkovskii and Schuurman, 2017). The frequency of the employed driving field sets an upper limit for the temporal window that can be covered in this type of experiment, which amounts to ≈1.6 fs for an 800-nm-field.…”