Quantum cascade laser spectroscopy of OCS isotopologues in 4He nanodroplets: A test of adiabatic following for a heavy rotor

Abstract

We report high-resolution infrared spectra of OCS isotopologues embedded in helium nanodroplets that were recorded with a newly built spectrometer. For the normal isotopologue, we observed the relatively weak third bending overtone band, in addition to new high J transitions in the C-O stretching fundamental, which has previously been investigated by diode laser spectroscopy [S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000)]. Similar to the gas phase, the overtone band is (only) 45 cm higher in energy than the fundamental, and this leads to additional broadening due to rapid vibrational relaxation that is accompanied by the creation of real/virtual phonon excitations. We also observed spectra in the C-O stretching fundamental for several minor isotopologues of OCS, including OCS, OCS, and OCS, in addition to some new peaks for OCS. A rovibrational analysis allowed for determination of the moment of inertia of helium (ΔI) that couples to the rotation of OCS for each isotopologue. In the context of the adiabatic following approximation, the helium density structure that follows the rotation of OCS should essentially remain unchanged between the isotopologues, i.e., there should be no dependence of ΔI on the gas phase moment of inertia of OCS (I). While this behavior was expected for the "heavy" OCS rotor investigated here, we instead found an approximately linear 1:1 relation between ΔI and I, which suggests partial breakdown of the adiabatic following approximation, making OCS the heaviest molecule for which evidence for this effect has been obtained.

The Effect of Infrared Light’s Power on the Infrared Spectra of Thin Films

“…For instance, our results justify the displacement in the minima of the absorption bands in Fourier transform IR spectra detected but not explained by other authors [14]. In addition, our model based on the law of conservation of energy agrees qualitatively and/or quantitatively with results presented in the current literature regarding Thomson scattering [3] [4], neutron star merger events [7], vibrational spectroscopy [2] [20] and optical manipulation of nanoparticles [9]. World Journal of Condensed Matter Physics bands do not change their location in different days.…”

“…Using Equation 2we estimate the energy transferred from the IR laser to the OCS molecule to be On the other hand, the evaporation energy per He atom of 0.83 meV, estimated by the authors of Ref. [20], seems very small. Indeed, for comparison, we recall that an energy of ~50 meV transmitted by visible light with P W µ to electrons in semiconductors is sufficient to cause each one of them to jump from shallow donor levels to the conduction band [21].…”

“…Experimental results of other authors help us to assess the validity of Equation (2). We will present two examples: one from Faulkner et al [20], and another by Reimann et al [9]. Faulkner et al [20] investigated the effects of a laser beam on the vib E of He nano-droplets at 9 K. These authors used an IR laser with an average power P of ~80 mW, and observed a resonance at 2000 cm −1 (6 × 10 13 Hz or 16.7 fs) with a ro-vibrational transition in a carbonyl sulphide (OCS) molecule accommodated in a He nano-droplet.…”

“…We will present two examples: one from Faulkner et al [20], and another by Reimann et al [9]. Faulkner et al [20] investigated the effects of a laser beam on the vib E of He nano-droplets at 9 K. These authors used an IR laser with an average power P of ~80 mW, and observed a resonance at 2000 cm −1 (6 × 10 13 Hz or 16.7 fs) with a ro-vibrational transition in a carbonyl sulphide (OCS) molecule accommodated in a He nano-droplet. Furthermore, the transfer of energy from the IR laser to the OCS molecule was observed to lead to the evaporation of ~400 He atoms in the nano-droplet.…”

Helium Nanodroplet Isolation Spectroscopy in an Undergraduate Teaching Laboratory

“…The helium nanodroplet isolation spectrometer the experiments were performed on was mostly built by students over the course of ~2 years. It has been described in detail elsewhere, [12][13] and here we provide only pertinent details; for additional details of the technique please see Refs. 6-8.…”