Excited Vibrational Levels Research Articles

Vibrationally induced decoherence in single-molecule junctions

We investigate the interplay of quantum interference effects and electronic-vibrational coupling in electron transport through single-molecule junctions, employing a nonequilibrium Green's function approach. Our findings show that inelastic processes lead, in general, to a quenching of quantum interference effects. This quenching is more pronounced for increasing bias voltages and levels of vibrational excitation. As a result of this vibrationally induced decoherence, vibrational signatures in the transport characteristics of a molecular contact may strongly deviate from a simple Franck-Condon picture. This includes signatures in both the resonant and the non-resonant transport regime. Moreover, it is shown that local cooling by electron-hole pair creation processes can influence the transport characteristics profoundly, giving rise to a significant temperature dependence of the electrical current.

A new approach toward transition state spectroscopy

Chirped-Pulse millimetre-Wave (CPmmW) rotational spectroscopy provides a new class of information about photolysis transition state(s). Measured intensities in rotational spectra determine species-isomer-vibrational populations, provided that the rotational populations can be thermalized. The formation and detection of S(0) vinylidene is discussed in the limits of low and high initial rotational excitation. CPmmW spectra of 193 nm photolysis of vinyl cyanide (acrylonitrile) contain J = 0-1 transitions in more than 20 vibrational levels of HCN and HNC, but no transitions in vinylidene or highly excited local-bender vibrational levels of acetylene. Reasons for the non-observation of the vinylidene co-product of HCN are discussed.

Static and dynamic polarizabilities of the non-relativistic hydrogen molecular ion

Static and dynamic polarizabilities are calculated for the non-relativistic hydrogen molecular ion by solving the three-body Schrödinger equation in perimetric coordinates with the Lagrange-mesh method. The static dipole polarizabilities of the ground-state rotational band are computed for total orbital (or rotational) angular momenta from L = 0 to 39 with an absolute accuracy of about 10−9 au. For L = 0, 1 and 2, slightly less accurate results are also given for the first three vibrational excited levels. For the ground state, dynamic dipole polarizabilities as well as static and dynamic quadrupole polarizabilities are computed. To illustrate the versatility of the method, the static dipole polarizability of the hydrogen molecular ion in a Debye plasma is also determined.

Fourier transform emission spectroscopy of the A2Π–X2Σ+ (red) system of 13C14N (II)

High resolution emission spectra of the A2Π–X2Σ+ transition of 13C14N have been measured in the 15000–24000cm−1 region. Molecules were produced by the reaction of 13CH4 and 14N2 in an active nitrogen afterglow discharge. The spectra were recorded using the Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the National Solar Observatory. A rotational analysis of 27 bands involving the excited state vibrational levels v′=9–22 and the ground state vibrational levels up to v″=12 has been obtained. An improved set of spectroscopic constants has been determined for the v=0–22 levels of the A2Π state by combining the present measurements with those reported previously for the v=0–8 vibrational levels of the A2Π state [Ram et al., Astrophys. J. Suppl. Ser. 188 (2010) 500] and existing infrared and millimeter-wave measurements of 13C14N. The 6–3, 7–4, 8–5 and 9–6 bands of the B2Σ+–A2Π transition were also identified in the 23300–24000cm−1 region and were included in the final analysis. An experimental line list and calculated term values are provided.

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.

Spontaneous Raman scattering: a useful tool for investigating the afterglow of nanosecond scale discharges in air

Nanosecond scale discharges are considered an interesting way for assisting combustion by enhancing either flame stabilization or ignition. Better understanding of energy deposit and radical species production processes is still required under pressure conditions normally encountered in combustion. The purpose of the present paper is to show that spontaneous Raman scattering, seldom used to investigate nanosecond pulsed discharges, is a useful measurement method for investigating the energy deposit of these discharges. The advantage of spontaneous Raman scattering is described by analyzing N2 and O2 spectra during the post-discharge of a filamentary nanosecond air discharge under atmospheric pressure, using phase-locked average spectra. The main advantages of spontaneous Raman scattering measurements are that they allow line-wise probing of different species with the same experimental setup and the determination of vibrational distribution by comparison with theoretical modeling over a wide range of vibrational levels (from v=0 to v=20 for N2). The model proposed takes into account the high level of vibrational excitation and the strong non-equilibrium observed, allowing the characterization of the vibrational relaxation over the complete post-discharge duration. Although the rotational structure is not resolved, the rotational temperature and thus translational temperature are determined with a moderate uncertainty for T above 500 K.

Local modes in vibration–rotation spectroscopy

AbstractThe present paper is concerned with the description of highly excited rotational and/or vibrational states of molecules in terms of localized vibrations or local modes. Local mode effects are most common in molecules with multiple equivalent H–X bonds, which give rise to equivalent H–X local mode stretching vibrations, and the theory is outlined for the simplest such case, that of an H2X molecule. In the local mode picture of molecular vibration, experimentally observed, initially unexpected near‐degeneracies of vibrational states at high vibrational excitation, and of rotation–vibration states at high rotational excitation, can be explained in a relatively straightforward manner. The local mode theory predicts relations between the conventional rotation–vibration parameters whose values are determined in least‐squares fittings to observed rotation–vibration transition frequencies or wavenumbers. It should be emphasized, however, that such relations are valid only for particular forms of the effective rotation–vibration Hamiltonian used in the spectral analysis. We illustrate the theory with examples of experimental spectroscopic work where local mode effects play an important role in the interpretation of the experimental findings. The fact that local mode vibrations not only cause clustering of highly excited vibrational energy levels, but also of highly rotationally excited rotation–vibration energy levels, has been understood fairly recently. We outline the theoretical background for this phenomenon and relate it to the existing experimental work. © 2012 John Wiley & Sons, Ltd.This article is categorized under: Theoretical and Physical Chemistry > Spectroscopy

ROVIBRATIONAL QUENCHING RATE COEFFICIENTS OF HD IN COLLISIONS WITH He

Along with H2, HD has been found to play an important role in the cooling of the primordial gas for the formation of the first stars and galaxies. It has also been observed in a variety of cool molecular astrophysical environments. The rate of cooling by HD molecules requires knowledge of collisional rate coefficients with the primary impactors, H, He, and H2. To improve knowledge of the collisional properties of HD, we present rate coefficients for the He-HD collision system over a range of collision energies from 10–5 to 5 × 103 cm–1. Fully quantum mechanical scattering calculations were performed for initial HD rovibrational states of j = 0 and 1 for v = 0-17 which utilized accurate diatom rovibrational wave functions. Rate coefficients of all Δv = 0, –1, and –2 transitions are reported. Significant discrepancies with previous calculations, which adopted a small basis and harmonic HD wave functions for excited vibrational levels, were found for the highest previously considered vibrational state of v = 3. Applications of the He-HD rate coefficients in various astrophysical environments are briefly discussed.

Photoelectron spectroscopic study of the E ⊗ e Jahn–Teller effect in the presence of a tunable spin–orbit interaction. II. Rovibronic analysis of the 2E ground state of CH3CI+

The PFI-ZEKE photoelectron spectrum of the CH3Cl+ E ← CH3Cl transition was recorded in the range 90900–93400 cm−1 following single-photon excitation from the ground state. The spectrum consists of well-resolved spin-vibronic bands, some of which with partially resolved rotational structure, and provides new information on the combined effects of the Jahn–Teller and spin–orbit interaction in the E ground state of CH3Cl+. The analysis of its spin-rovibronic structure reveals a weak linear Jahn–Teller effect along the H3C–Cl bending e mode (ν6) and enabled the derivation of Jahn–Teller coupling constants along ν6, rotational constants, and spin–orbit interaction parameters for the E ground state of CH3Cl+. The Jahn–Teller effect is dynamical in the ground state of CH3Cl+ and the photoelectron spectrum does not provide evidence for a reduction of the molecular symmetry to Cs symmetry. Weak rotational satellite bands were observed in transitions to several excited vibrational levels of CH3Cl+ and interpreted in terms of the spin-rovibronic photoionization selection rule derived in the analysis of the photoelectron spectrum of CH3I (M. Grütter, J. M. Michaud and F. Merkt, J. Chem. Phys. 134, 054308 (2011)). Compared to the ground state of HCl+ ( cm−1), the spin–orbit splitting is strongly reduced to cm−1. The reduction is primarily attributed to an electronic effect and, to a lesser extent, to the quenching of the spin–orbit interaction by the Jahn–Teller effect .

Stark-induced adiabatic Raman passage for preparing polarized molecules

We propose a method based on Stark-induced adiabatic Raman passage (SARP) for preparing vibrationally excited molecules with known orientation and alignment for future dynamical stereochemistry studies. This method utilizes the (J, M)-state dependent dynamic Stark shifts of rovibrational levels induced by delayed but overlapping pump and Stokes pulses of unequal intensities. Under collision-free conditions, our calculations show that we can achieve complete population transfer to an excited vibrational level (v > 0) of the H(2) molecule in its ground electronic state. Specifically, the H(2) (v = 1, J = 2, M = 0) level can be prepared with complete population transfer from the (v = 0, J = 0, M = 0) level using the S(0) branch of the Raman transition with visible pump and Stoke laser pulses, each polarized parallel to the z axis (uniaxial π-π Raman pumping). Similarly, H(2) (v = 1, J = 2, M = ±2) can be prepared using SARP with a left circularly polarized pump and a right circularly (or vice versa) polarized Stokes wave propagating along the z axis (σ(±)-σ(∓) Raman pumping). This technique requires phase coherent nanosecond pulses with unequal intensity between the pump and the Stokes pulses, one being four or more times greater than the other. A peak intensity of ~16 GW/cm(2) for the stronger pulse is required to generate the desirable sweep of the Raman resonance frequency. These conditions may be fulfilled using red and green laser pulses with the duration of a few nanoseconds and optical energies of ~12 and 60 mJ within a focused beam of diameter ~0.25 mm. Additionally, complete population transfer to the v = 4 vibrational level is predicted to be possible using SARP with a 355-nm pump and a near infrared Stokes laser with accessible pulse energies.

Vibrational calculations in formaldehyde: the CH stretch system

Abstract An alternative procedure for the calculation of highly excited vibrational levels in S0 formaldehyde was developed to apply to larger molecules. It is based on a new set of symmetrized vibrational valence coordinates. The fully symmetrized vibrational kinetic energy operator is derived in these coordinates using the Handy expression [Molec. Phys. 61, 207 (1987)]. The potential energy surface is expressed as a fully symmetrized quartic expansion in the coordinates. We have performed ab initio electronic computations using GAMESS to obtain all force constants of the S0 formaldehyde quartic force field. Our large scale vibrational calculations are based on a fully symmetrized vibrational basis set, in product form. The vibrational levels are calculated one by one using an artificial intelligence search/selection procedure and subsequent Lanczos iteration, providing access to extremely high vibrational energies. In this work special attention has been given to the CH stretch system by calculating the energies up to the fifth CH stretch overtone at ∼16000 cm−1, but the method has also been tested on two highly excited combination levels including other lower frequency modes.

Near threshold Cl−·CH3I photodetachment: Apparent 2P1/2 channel suppression and enhanced 2P3/2 channel vibrational excitation

Cl(-)·CH(3)I cluster anion photoelectron images are recorded over a range of detachment wavelengths in the immediate post threshold region. The photoelectron spectral features fall into two categories. A number of weak, photon energy dependent transitions are observed and attributed to atomic anion fragmentation products. Several more intense, higher electron binding energy transitions result from single photon cluster anion detachment. Comparison with I(-)·CH(3)I suggests that the detachment process is more complicated for Cl(-)·CH(3)I. The single photon transition spacing is consistent with CH(3)I ν(3) mode excitation, but the two distinct vibronic bands of I(-)·CH(3)I detachment are not easily distinguished in the Cl(-)·CH(3)I spectra. Similarly, while the spectral intensities for both cluster anions show non-Franck Condon behavior, the level of vibrational excitation appears greater for Cl(-)·CH(3)I detachment. These observations are discussed in terms of low lying electronic states of CH(3)I along the C-I coordinate, and the influence of the CH(3)I moiety on the neutral halogen atom states.

VIBRATIONALLY EXCITED C 6 H

Rotational spectra in four new excited vibrational levels of the linear carbon chain radical C4H were observed in the millimeter band between 69 and 364 GHz in a low pressure glow discharge, and two of these were observed in a supersonic molecular beam between 19 and 38 GHz. All have rotational constants within 0.4% of the ground vibrational state of C4H and were assigned to new bending vibrational levels, two each with and vibrational symmetry. The new levels are tentatively assigned to the and bending vibrational modes (both with symmetry), and the and combination levels ( symmetry) on the basis of the derived spectroscopic constants, relative intensities in our discharge source, and published laser spectroscopic and quantum calculations. Prior spectroscopic constants in the and levels were refined. Also presented are interferometric maps of the ground state and the level obtained with the Submillimeter Array (SMA) near 257 GHz which show that C4H is present near the central star in IRC+10216. We found no evidence with the SMA for the new vibrationally excited levels of C4H at a peak flux density averaged over a synthesized beam of Jy/beam in the 294–296 and 304–306 GHz range, but it is anticipated that rotational lines in the new levels might be observed in IRC+10216 when ALMA attains its full design capability.

Direct determination of state-to-state rotational energy transfer rate constants via a Raman-Raman double resonance technique: ortho-acetylene in v2=1 at 155 K

A new technique for the direct determination of state-to-state rotational energy transfer rate constants in the gas phase is presented. It is based on two sequential stimulated Raman processes: the first one prepares the sample in a single rotational state of an excited vibrational level, and the second one, using the high resolution quasi-continuous stimulated Raman-loss technique, monitors the transfer of population to other rotational states of the same vibrational level as a function of the delay between the pump and the probe stages. The technique is applied to the odd-J rotational states of v(2)=1 acetylene at 155 K. The experimental layout, data acquisition, retrieval procedures, and numerical treatment are described. The quantity and quality of the data are high enough to allow a direct determination of the state-to-state rate constant matrix from a fit of the experimental data, with the only conditions of detailed balance and of a closed number of states. The matrix obtained from this direct fit is also compared with those obtained using some common fitting and scaling laws.

Processes in high-temperature air involving molecules and atoms in excited electron states

Analysis is made of kinetic processes involving electron states of atoms and molecules in the air behind the shock wave front. The electron states of atoms and molecules are formed in high-temperature air as a result of dissociation and recombination, in the processes of electron exchange of energy in collisions of particles, and in chemical exchange reactions. The fastest one of these processes is the process of formation of excited electron states under conditions of recombination of atoms and during isenergic transition of vibrational-to-electron energy from highly excited vibrational levels. The quenching of metastable particles occurs in collisions of particles, under conditions of dissociation and recombination, and in chemical exchange reactions. The data base for rate constants of electron energy exchange reactions in high-temperature air is demonstrated. Examples are given of description of vibrational energy exchange in electron-chemical reactions.

CCSD(T) study of the infrared spectrum of ethyl-methyl-ether isotopic varieties

Abstract Band positions for the infrared bands of various ethyl-methyl-ether isotopomers (CH3CH2OCH2D, CH2DCH2OCH3, CH3CH2OCD3, CD3CH2OCH3, CH3CD2OCH3, CH3CH2O13CH3, 13CH3CH2OCH3, and CH313CH2OCH3) are determined using second order perturbation theory. For species showing G18 symmetry, band position are calculated variationally from a CCSD(T)/cc-pVTZ three-dimensional potential energy surface corrected vibrationally. Potential energy barriers, fundamental frequencies, and rotational constants for excited vibrational levels, are also provided. Calculated frequencies for CH3CH2OCD3 confirm experimental assignments and our predictions for the most abundant isotopomer [4] .

Vibrational spectroscopy of C m -C/C b -C b stretching vibrations of copper tetramesityl porphyrin: An algebraic approach

Using Lie algebraic techniques and simpler expressions of the matrix elements of Majorana and Casimir operators given by us, we obtain an effective Hamiltonian operator which conveniently describes stretching vibrations of biomolecules. For a copper tetramesityl porphyrin molecule, the higher excited vibrational levels are calculated by applying the U(2) algebraic approach.

The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band (286-333 nm) has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state (nu=0). The kinetic energy distributions (KED) of the produced CH(3)(nu=0) fragments show a vibrational structure, both in the I((2)P(3/2)) and I( *)((2)P(1/2)) channels, due to the contribution to the overall process of initial vibrational excitation in the nu(3)(C-I) mode of the parent CH(3)I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH(3)I when the photolysis wavelength is increased. The I((2)P(3/2))/I( *)((2)P(1/2)) branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH(3)I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, (3)Q(0), (1)Q(1), and (3)Q(1), as well as the probability of nonadiabatic curve crossing (1)Q(1)<--(3)Q(0). The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality (pseudotriatomic) model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I((2)P(3/2))/[I((2)P(3/2))+I( *)((2)P(1/2))] branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies.

Electronic energy exchange in high-temperature air

Kinetic processes with the participation of electronic states of atoms and molecules in air behind the shock wave front were analyzed. All metastable levels of molecular and atomic oxygen and nitrogen and nitrogen oxide molecules situated below the dissociation energy were analyzed. In high-temperature air, atom and molecule electronic states are formed in dissociation and recombination, electronic energy exchange in collisions of particles, and chemical exchange reactions. The formation of excited electronic states in the recombination of atoms and isoenergy vibrational energy transfer from highly excited vibrational levels into electronic energy is the fastest process. The quenching of metastable particles occurs in collisions between particles, dissociation and recombination, and chemical exchange reactions. A database on electronic energy exchange rate constants in high-temperature air is presented.

Design of Infrared Laser Pulses for the Vibrational De-excitation of Translationally Cold Li2 Molecules

In connection with attempts to form molecular Bose-Einstein condensates, there have been reports in the literature of the preparation of samples of translationally cold alkali metal dimers. The molecules in these samples are generally in excited vibrational levels. To form a stable Bose-Einstein condensate, the molecules must be de-excited to their lowest vibrational state. In this paper, we demonstrate that through the use of optimal control theory, it is possible to design a sequence of infrared laser pulses that will de-excite a sample of (7)Li2 molecules from the v = 10 vibrational level of their (1)Sigma(g)+ ground electronic state to their lowest v = 0 vibrational level with an overall efficiency of 91.1%.