Molecular vibration

Summary

A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 1013 Hz to approximately 1014 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm−1 and wavelengths of approximately 30 to 3 µm. For a diatomic molecule A−B, the vibrational frequency in s−1 is given by \nu = \frac{1}{2 \pi} \sqrt{k / \mu} , where k is the force constant in dyne/cm or erg/cm2 and μ is the reduced mass given by \frac{1}{\mu} = \frac{1}{m_A}+\frac{1}{m_B}. The vibrational wavenumber in cm−1 is \tilde{\nu} ;= \frac{1}{2 \pi c} \sqrt{k / \mu}, where c is the speed of light in cm/s. Vibrations of polyatomic molecules are described in terms of normal modes, which are independent of each other, but each normal mode involves simultaneous vibrations of

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https://en.wikipedia.org/wiki/Molecular_vibration

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In this thesis, I present an experimental molecular beam surface scattering study of dynamical processes involved when HCl molecules are scattered from Au(111) and Ag(111) surfaces. I investigated vibrational excitation, translational inelasticity and dissociative adsorption in combination with associative desorption. The experiments were conducted in an ultra-high vacuum (UHV) molecular beam/surface scattering apparatus equipped with a pulsed nanosecond infrared (IR) laser source for vibrational state manipulation and pulsed nanosecond ultraviolet (UV) lasers for quantum-state-resolved detection of molecules via resonance enhanced multi-photon ionization (REMPI) before and after the collisions. For HCl/Au(111), I found surface temperature dependent vibrational excitation probabilities (VEPs) from vibrational state v = 0 -> 1 to be in the range of 10^-5 - 10^-3 for incidence energies of Ei = 0.67 - 0.99 eV, which is low compared to other molecule-surface systems. On the other hand, VEPs for the v = 1 -> 2 transition were substantially higher at 10^-3 - 10^-2 for comparable incidence energies. In both cases, excitation probabilities could be divided into electronically adiabatic and nonadiabatic contributions where the latter exponentially depended on the surface temperature Ts. This combination of adiabatic and nonadiabatic vibrational excitation has so far been uniquely observed for HCl scattered from Au(111) and Ag(111) surfaces. Extracting Ei and Ts independent interaction coefficients, I found the nonadiabatic excitation to be stronger than the adiabatic one by a factor of 67 for the v = 0 -> 1 and 24 for the v = 1 -> 2 channel. In comparison, on Ag(111) only the excitation from the vibrational ground state could be observed. With VEPs in the range of 10^-4 - 10^-3 being slightly higher than on gold, this difference was interpreted as enhanced nonadiabatic interactions on silver. Relatively low barriers to dissociation on both metal surfaces (e. g., compared to NO/Au(111)) might have led to the higher v = 1 -> 2 VEPs on Au(111). Further, the absence of higher excitation channels (v = 2 -> 3 on gold, v = 1 -> 2 on silver) might also be explained by the enhanced dissociation of HCl molecules incident in excited vibrational states. Dissociation probabilities of HCl on metal surfaces were measured by AUGER electron spectroscopy after controlled molecular beam dosage monitored by a quadrupole mass spectrometer. Even though the dissociation probabilities on Ag(111) were predicted to be higher than on Au(111), I could not determine them within the current technical limitations. Additionally, the actual dissociation probabilities I determined on Au(111) employing AES remained below 0.06 - 0.17, which is lower than those predicted by a series of computational studies by a factor of 2 - 6. On the other hand, translational energy losses calculated in the latest AIMDEF study match the experimental results for v = 1 -> 1 and v = 1 -> 2 quite accurately. In sum, the results of this thesis, which all indicate that HCl exhibits both electronically adiabatic and nonadiabatic interactions with metal surfaces during scatter- ing events, can serve as benchmark data to test and improve theoretical descriptions of gas-surface interactions, especially in those cases were nonadiabaticity plays an important role.

EPFL2019

With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing.This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâs transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 Â± 0.025, violating the CHSH inequality, compatible with the non-locality of the state.The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.

EPFL2021

Infrared photofragment spectroscopy of charged amino acid water clusters in the gas phase

Anthi Kamariotou

We present in this work a new technique, which combines laser photofragmentation spectroscopy with tandem mass spectrometry, for structural investigations of biomolecular ions in the gas phase. A novel apparatus was designed and built for the implementation of these studies. In this instrument closed shell ions are produced in the gas phase by electrospray ionization and stored in a hexapole ion trap prior to mass selection of a parent beam in a first quadrupole and then irradiation in an octupole ion guide using laser light. Mass analysis of the photofragmentaion products in a final stage quadrupole as a function of the wavelength generates an action spectrum. We applied this new technique to follow the microsolvation of charged amino acids in the gas phase. In these studies, the nanoelectrospray ionization source generates a distribution of water clusters of charged amino acids at various hydration levels. A particular size of cluster is selected and irradiated by infrared laser pulses resulting in the dissociation of one water molecule. Detection of the photofragmentation of this weak non-covalent bond allows us to generate the vibrational action spectrum of this particular cluster. We use a two-stage difference frequency mixing setup to produce laser light in the 2900–3800 cm-1, allowing us to probe the light-atom stretching region. IR photofragmentation spectra have been obtained for the hydrates of protonated and lithiated valine and those of protonated tryptophan. By probing the region of free and hydrogen-bonded N-H and O-H stretch vibrations and with the help of density functional theory calculations at the B3LYP/6-31++G** level, we relate spectral changes to the structure of clusters. In the study of lithiated valine water clusters, we addressed the question of zwitterion formation upon the combined effect of water and an external ion. Our data indicate the presence of the non-zwitterionic form of valine upon addition of up to four water molecules. In all of the species studied here, the hydration process is driven by solvation of the charge, and upon completion of a first shell around it, water preferentially forms a second solvation shell with no strong competition from other hydration sites on the amino acid backbone. Strikingly, similar water network structures have been observed at the highest hydration level of completely different species, probably indicating the existence of stable ordered water structures. We obtained evidence for a structrual change of the valine backbone in lithiated valine upon addition of the third water molecule, while no conformational change has been identified in the clusters of the protonated species. We have thus been able to answer questions related to the conformational preferences of the amino acid and the structuring of the water network.

EPFL2006

EPFL2021

EPFL2019

Infrared photofragment spectroscopy of charged amino acid water clusters in the gas phase

Anthi Kamariotou

EPFL2006