Amplification of Anharmonicities in Multiphoton Vibrational Action Spectra

Abstract

The influence of one or several infrared laser pulses on the stability of bare and argon-tagged sodium chloride clusters is investigated theoretically by a combination of computational methods involving explicit molecular dynamics and properly calibrated unimolecular rate theories. The fragmentation spectra obtained by varying the laser frequency in the far-IR range is compared to the linear absorption spectrum resulting from the dipole moment autocorrelation function. Under appropriate laser field parameters, the action spectra are found to resemble the absorption spectra quite accurately in terms of positions, line widths, and even relative intensities. However, the action spectra exhibit residual and systematic redshifts of a few percent, which are partly due to the finite spectral bandwidth but are amplified by the progressive heating by the laser. A quantitative analysis suggests that these anharmonicity effects should generally arise upon multiple photon absorption.

Atomistic Modeling of IR Action Spectra Under Circularly Polarized Electromagnetic Fields: Toward Action VCD Spectra

“…Compared with the harmonic spectrum, the bands in the action spectrum thus deviate even more than the anharmonic spectrum at the same initial temperature. This computational observation confirms the amplification mechanism previously identified as the cause of these apparent excessive band shifts. More precisely, the additional shifting originates from the heating of the system while still under exposure to the laser, the resonance frequencies ω being altered by temperature due to anharmonicities, at first corrective order as ω ( T ) ≅ ω ( T = 0) × (1 + a 1 T + a 2 T 2 + ⋯) with a 1 , a 2 , … some constant parameters .…”

“…The general protocol of our simulations has been laid out in earlier papers in the case of linearly polarized laser pulses and it will only be sketched here. Briefly, we follow a purely classical modeling in which the molecule of interest is initially prepared at a required temperature (step 1), and subjected at time t = 0 to a single electromagnetic pulse at fixed frequency ω in the infrared range, with duration τ (step 2).…”

“…Anharmonicities are thus essential in gas phase spectroscopy, as they are responsible for the energy redistribution between the excited and dissociating modes. Furthermore, in multiphoton excitation processes such as those at play in IR‐MPD or in IR resonance‐enhanced multiphoton ionization (IR‐REMPI) nonlinear effects may occur due to the competition between dynamic processes involving the absorption, relaxation, and statistical decay, all with their own intrinsic time scales …”

A simple but accurate potential for the naphthalene-argon complex: Applications to collisional energy transfer and matrix isolated IR spectroscopy

“…Recent approaches based on time multiscale techniques 87 indicate that spectral features may be altered (broadened and redshifted) as a consequence of the combined effects of anharmonicities and dynamical heating in the laser field. 88 In view of the importance of naphthalene and larger PAHs in the astrophysical context, 89, 90 a realistic determination of naphthalene-argon predissociation spectra could be useful for assisting future laboratory measurements by action spectroscopy.…”

Nonlinear effects in infrared action spectroscopy of silicon and vanadium oxide clusters: experiment and kinetic modeling

“…33 Except these investigations, we are not aware of earlier attempts to model the dependence of IR spectra of gas phase species on the experimental conditions. Recently, we made a description of nonlinear effects in vibrational action spectroscopy from a molecular perspective, either directly at the atomistic level through explicit time-dependent molecular dynamics 34 or from a discrete quantum mechanical description of energy levels. 35 Although these approaches rely on different assumptions and make use of a complementary treatment for the nuclear dynamics and the interaction with the laser field, they consistently predict action spectra that vary with the excitation pulse and systematically differ from the linear absorption spectra even after anharmonicities are accounted for.…”