Quantum initial value representation simulation of water trimer far infrared absorption spectrum

We extend the technique of quantum propagation on a grid of trajectory guided coupled coherent states to simulate experimental absorption spectra. The approach involves calculating the thermally averaged dipole moment autocorrelation function by means of quantum propagation in imaginary time. The method is tested on simulation of the far infrared spectrum of water trimer based on a three-dimensional model potential. Results are in good agreement with experiment and with other calculations.     

Direct infrared absorption spectroscopy of benzene dimer

The direct infrared (IR) absorption spectrum of benzene dimer formed in a free-jet expansion was recorded in the 3.3 μm region for the first time. This has led to the observation of the C-H stretching fundamental mode ν(13) (B(1u)), which is both IR and Raman forbidden in the monomer. Moreover, the IR forbidden and Raman allowed ν(7) (E(2g)) mode has been observed as well. These two modes were found to be red-shifted along with the IR allowed ν(20) (E(1u)) mode, as previously reported by Erlekam et al. [Erlekam; Frankowski; Meijer; Gert von Helden J. Chem. Phys.2006, 124, 171101], using ion-dip spectroscopy, contrary to the blue-shift predicted earlier by theoretical studies. The observation of the ν(13) band indicates that the symmetry is reduced in the dimer, confirming the T-shaped structure observed by Erlekam et al. Our experimental results have not provided any direct evidence for the presence of the parallel displaced geometry, the main objective of the present work, as predicted by theoretical calculations.     

The absorption spectrum of water near 750 nm by CW-CRDS: contribution to the search of water dimer absorption

The absorption spectrum of natural water vapour around 750 nm has been recorded with a typical sensitivity of 3 x 10(-10) cm(-1) using a cw cavity ring down spectroscopy set up based on a Ti:sapphire laser. The 13 312.4-13 377.7 cm(-1) spectral interval was chosen as it corresponds to the region where water dimer absorption was recently measured (K. Pfeisticker et al., Science, 2003, 300, 2078-2080). The line parameters (wavenumber and intensity) of a total of 286 lines of water vapor were measured by a one by one fit of the lines to a Voigt profile. For the main water isotopologue, 276 lines were measured with line intensities as weak as 5 x 10(-29) cm molecule(-1)i.e. about 50 times smaller than the weakest H(2)16O line intensities included in the 2004 edition of the HITRAN database. On the basis of the predictions of Schwenke and Partridge, all but 16 lines could be assigned to different isotopologues of water (H(2)16O, H(2)18O, and HD16O) present in natural abundance in the sample. A total of 272 energy levels of H(2)16O were determined and rovibrationally assigned to 18 upper vibrational states. Half of them had not been reported previously. The importance of the additional absorbance resulting from the observation of many new weak lines is discussed in relation to the detection of water dimer absorption and compared to the absorbance predicted by Schwenke and Partridge. The quality of the line parameters of water monomer is shown to be of crucial importance to identify the absorbance of the water dimer in the considered region.     

A single line,far infrared water laser

Lasing at 285.55 cm^?1 has been observed from a flash photolyzed H₂ and O₃ mixture. The oscillating line has been assigned to a water transition pumped by a resonance energy transfer mechanism.     

A comparison of light scattering and far infrared absorption spectra of simple liquids

Light scattering and far infrared absorption spectra of CCl₄, C₂Cl₄, C₆H₁₂, CHCl₃, and CH₂Cl₂ in the liquid phase have been obtained in the range of 2 – 200 and 20 – 200 cm^?1 respectively. The energy absorption spectra obtained by the two techniques and the corresponding relaxation times were compared for each liquid. We observe systematic differences between the energy absorption profiles obtained from the light scattering spectra and the far infrared absorption spectras. We also find generally shorter relaxation times from the infrared absorption spectra. Despite the similarity of the physical processes leading to light scattering and to far infrared absorption some significant differences are observed (ref. 1,2).     

An ab initio calculation of the absorption cross section of water in the first absorption continuum

We present a complete ab initio study of the photodissociation of water in the first absorption band including a calculated excited-state potential and a calculated transition dipole function. The dynamical treatment is based on the IOS approximation for the rotational degree of freedom. The vibrationally coupled equations are solved exactly using polar coordinates. Agreement with the experimental absorption spectrum is excellent and even the “vibrational” structures are reproduced. A dynamical interpretation of these structures in terms of features of the à ¹B₁ surface is given.     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

A model approach to far infrared absorption in strongly polar fluids: Clusters and their internal angular modes

It is suggested that highly polar molecules tend to aggregate into microclusters of various configurations. For neat liquids at high densities such clustering might account for the strong orientational correlations which have been observed in numerical simulations. Three examples are given to illustrate the far infrared spectroscopic properties of such microclusters: the fundamental modes of libration of linear “dimer” and “trimer”, and an adiabatic rotational mode in a seven dipoles cluster. In the latter example the rotational motions are concerted so that no potential barrier exist along a complete rotational cycle; consequently the corresponding absorption should extend over the thermal free rotation spectral region. Such an absorption has been recently observed by F.I.R. spectroscopy in liquid methyl chloride.     

Contribution to the development of atmospheric radioxenon monitoring

Within the frame of Comprehensive Nuclear-Test Ban Treaty (CTBT), this paper deals with the development of the new techniques necessary for the xenon monitoring requested by the CTBT. An automatic system called SPALAX™, devoted to the on-site sampling and measurement was developed by French atomic energy commission (CEA). Analytical methods and equipments have been studied at our laboratory, using dual X-γ-spectrometry in order to get independent means with better sensitivity within a robust quality assurance program. In the case of a wide number of potential existing sources and depending on meteorological conditions, several solutions can be arrived at.     

Effect of structural characteristics of water on the infrared absorption and raman spectra

An analysis of band contour in the IR absorption (overtone region) and Raman spectra of water suggests that the high-frequency components of spectral bands correspond to a single continuous distribution in energy of H-bonds. A local increase in the probability density function at high frequencies, which leads to an increase in the population of states with weak hydrogen bonds, is caused by the specific behavior of the angular dependence of the energy of hydrogen bonds between water molecules. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 467–472, May–June, 1995. Translated by I. Izvekova     

The far infrared spectrum of nitroethane

The far infrared spectrum of nitroethane measured by Founer spectroscopy in the 8-60 cm^?1 is presented and interpreted in terms of a rigid frame-rigid top model for free internal rotation group. Ratational-internal rotational energy levels and absolute absorption intensities are calculated to first order for an optimal choice of molecule-fixed axes. The computed absorption based on a permanent dipole moment of 3.4 debye along the NC direction is in excellent agreement with the observed band.     

State-resolved infrared spectrum of the protonated water dimer: revisiting the characteristic proton transfer doublet peak

The infrared (IR) spectra of protonated water clusters encode precise information on the dynamics and structure of the hydrated proton. However, the strong anharmonic coupling and quantum effects of these elusive species remain puzzling up to the present day. Here, we report unequivocal evidence that the interplay between the proton transfer and the water wagging motions in the protonated water dimer (Zundel ion) giving rise to the characteristic doublet peak is both more complex and more sensitive to subtle energetic changes than previously thought. In particular, hitherto overlooked low-intensity satellite peaks in the experimental spectrum are now unveiled and mechanistically assigned. Our findings rely on the comparison of IR spectra obtained using two highly accurate potential energy surfaces in conjunction with highly accurate state-resolved quantum simulations. We demonstrate that these high-accuracy simulations are important for providing definite assignments of the complex IR signals of fluxional molecules.

We reveal the intricate dynamics of the proton shuttling motion in the Zundel ion by computing 900 high-accuracy vibrational eigenstates. We show how very subtle energetic changes in the vibrational modes lead to vastly different infrared spectra.     

Vibronic absorption spectrum of the ethylene dimer

Vibronic coupling theory is used to construct the vibronic absorption spectrum of the ethylene dimer. It is shown that in this case an extended four-parameter form of the vibronic Hamiltonian should be considered. In addition to the commonly used three vibronic parameters, the difference between the ground and excited state force constants of the monomer is taken into account. Numerical calculations were performed for the dimer geometry resembling that of norbornadiene. Some comments on the interpretation of the absorption spectrum of norbornadiene are made.     

Molecular dynamics simulations for CO2 spectra. II. The far infrared collision-induced absorption band

Classical molecular dynamics simulations have been carried out for gaseous CO(2) starting from various anisotropic intermolecular potential energy surfaces. Through calculations for a large number of molecules treated as rigid rotors, the time evolution of the interaction-induced electric dipole vector is obtained and the Laplace transform of its autocorrelation function gives the collision-induced absorption rototranslational spectrum. The results are successfully compared with those of previous similar calculations before studies of the influences of the intermolecular potential and induced-dipole components are made. The calculated spectra show a significant sensitivity to anisotropic forces consistently with previous analyses limited to the spectral moments. The present results also demonstrate the importance of vibrational and back-induction contributions to the induced dipole. Comparisons between measured far infrared (0-250 cm(-1)) spectra at different temperatures and results calculated without the use of any adjustable parameter are made. When the best and more complete input data are used, the quality of our predictions is similar to that obtained by Gruszka et al. [Mol. Phys. 93, 1007 (1998)] after the introduction of ad hoc short-range overlap contributions. Our results thus largely obviate the need for such contributions the magnitudes of which remain questioned. Nevertheless, problems remain since, whereas good agreements with measurements are obtained above 50 cm(-1), the calculations significantly underestimate the absorption below, a problem which is discussed in terms of various possible error sources.     

The far infrared spectrum of pyridine n-oxide

The two lowest internal modes of pyridine N-oxide and three of its lattice oscillations in the solid state at room temperature are reported for the first time on the basis of careful far infrared investigations from 33 to 525 cm^?1.     

Ellipsometric studies related to surface-enhanced infrared absorption

Infrared ellipsometry has been applied to determine the refractive index n and the absorption index k as well as the thickness of metal island films causing surface enhanced infrared absorption (SEIRA). The results from numerous films prepared in several campaigns are presented. For films of a nominal thickness of 6 nm, k is found to range from 0 to 3, while n ranges from 5 to 8. Among these two optical constants a close correlation is observed. Layers suitable for SEIRA exhibit an unusual spectral feature whose origin is explained.