Population Of Vibrational States Research Articles

Linear response in the strong-field domain: ultrafast wave packet interferometry in the continuum

We demonstrate that there are cases in which the response of a molecular system to the effects of a strong laser field is linear. By numerically solving the quantum dynamics of a wave packet of photo-dissociated H, we show that the initial state depletion and the photodissociation yields to various final channels may at times be an essentially linear function of the laser peak intensity. We have also investigated the case when the excitation is performed with delayed pulses, where the linear response regime is accompanied by oscillations in the population of vibrational and rotational states as a function of the delay time. In keeping with the observation of linear response beyond the weak field perturbative limit, coherent control over branching ratios, which can only exist in the strong field regime, is also shown to be extensive.

Communication: Transfer of more than half the population to a selected rovibrational state of H2 by Stark-induced adiabatic Raman passage

By using Stark-induced adiabatic Raman passage (SARP) with partially overlapping nanosecond pump (532 nm) and Stokes (683 nm) laser pulses, 73% ± 6% of the initial ground vibrational state population of H(2) (v = 0, J = 0) is transferred to the single vibrationally excited eigenstate (v = 1, J = 0). In contrast to other Stark chirped Raman adiabatic passage techniques, SARP transfers population from the initial ground state to a vibrationally excited target state of the ground electronic surface without using an intermediate vibronic resonance within an upper electronic state. Parallel linearly polarized, co-propagating pump and Stokes laser pulses of respective durations 6 ns and 4.5 ns, are combined with a relative delay of ~4 ns before orthogonally intersecting the molecular beam of H(2). The pump and Stokes laser pulses have fluences of ~10 J/mm(2) and ~1 J/mm(2), respectively. The intense pump pulse generates the necessary sweeping of the Raman resonance frequency by ac (second-order) Stark shifting the rovibrational levels. As the frequency of the v = 0 → v = 1 Raman transition is swept through resonance in the presence of the strong pump and the weaker delayed Stokes pulses, the population of (v = 0, J = 0) is coherently transferred via an adiabatic passage to (v = 1, J = 0). A quantitative measure of the population transferred to the target state is obtained from the depletion of the ground-state population using 2 + 1 resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The depletion is measured by comparing the REMPI signal of (v = 0, J = 0) at Raman resonance with that obtained when the Stokes pulse is detuned from the Stark-shifted Raman resonance. No depletion is observed with either the pump or the Stokes pulses alone, confirming that the measured depletion is indeed caused by the SARP-induced population transfer from the ground to the target state and not by the loss of molecules from photoionization or photodissociation. The two-photon resonant UV pulse used for REMPI detection is delayed by 20 ns with respect to the pump pulse to avoid the ac Stark shift originating from the pump and Stokes laser pulses. This experiment demonstrates the feasibility of preparing a large ensemble of isolated molecules in a preselected single quantum state without requiring an intermediate vibronic resonance.

Dynamics of Functionalized Surface Molecular Monolayers Studied with Ultrafast Infrared Vibrational Spectroscopy.

The structural dynamics of thin films consisting of tricarbonyl (1,10-phenanthroline)rhenium chloride (RePhen(CO)(3)Cl) linked to an alkyl silane monolayer through a triazole linker synthesized on silica-on-calcium-fluoride substrates are investigated using ultrafast infrared (IR) techniques. Ultrafast 2D IR vibrational echo experiments and polarization selective heterodyne detected transient grating (HDTG) measurements, as well as polarization dependent FT-IR and AFM experiments are employed to study the samples. The vibrational echo experiments measure spectral diffusion, while the HDTG experiments measure the vibrational excited state population relaxation and investigate the vibrational transition dipole orientational anisotropy decay. To investigate the anticipated impact of vibrational excitation transfer, which can be caused by the high concentration of RePhen(CO)(3)Cl in the monolayer, a concentration dependence of the spectral diffusion is measured. To generate a range of concentrations, mixed monolayers consisting of both hydrogen terminated and triazole/RePhen(CO)(3)Cl terminated alkyl silanes are synthesized. It is found that the measured rate of spectral diffusion is independent of concentration, with all samples showing spectral diffusion of 37 ± 6 ps. To definitively test for vibrational excitation transfer, polarization selective HDTG experiments are conducted. Excitation transfer will cause anisotropy decay. Polarization resolved heterodyne detected transient grating spectroscopy is sensitive to anisotropy decay (depolarization) caused by excitation transfer and molecular reorientation. The HDTG experiments show no evidence of anisotropy decay on the appropriate time scale, demonstrating the absence of excitation transfer the RePhen(CO)(3)Cl. Therefore the influence of excitation transfer on spectral diffusion is inconsequential in these samples, and the vibrational echo measurements of spectral diffusion report solely on structural dynamics. A small amount of very fast (~2 ps time scale) anisotropy decay is observed. The decay is concentration independent, and is assigned to wobbling-in-a-cone orientational motions of the RePhen(CO)(3)Cl. Theoretical calculations reported previously for experiments on a single concentration of the same type of sample suggested the presence of some vibrational excitation transfer and excitation transfer induced spectral diffusion. Possible reasons for the experimentally observed lack of excitation transfer in these high concentration samples are discussed.

Ab initio study of band strengths for the F 2Σ +–A 2Π electronic transition of AlO

Ab initio calculations of band strengths have been carried out for the F 2Σ +–A 2Π electronic transition of the AlO molecule. The band strength for the 0-0 band, the only band observed, is the largest of the 56 calculated bands. The band strengths for unobserved 19 bands are of the same order of magnitude as the 0-0 band, including the 1-0 and 2-1 bands whose observations are expected to lead to experimental determination of the vibrational constants for the F 2Σ + state. The Einstein coefficients for spontaneous emission of the F–A bands have been evaluated from the present band strengths and transition energies, and are used to estimate the ratios of the populations in those initial vibrational states, which produce 1-0 and 2-1 bands not weaker in intensity than the detectable 0-0 band, to the population that produces the detectable band.

Combination of micro- and macroscopic descriptions of the kinetics of chemical reactions and processes of vibrational energy exchange in a multicomponent gas mixture

Chemical and vibrational kinetics equations for a multicomponent reacting gas mixture constructed by combining microscopic (level-by-level) and macroscopic (mode) descriptions are presented. A mixture of diatomic molecules, anharmonic oscillators, which interact with each other and other generally polyatomic components is treated as a subsystem described microscopically, that is, in the form of equations of balance of vibrational state populations. The subsystem of polyatomic components is considered macroscopically, that is, in the form of equations for component concentrations and mean vibrational mode energies. The kinetic equations obtained this way for a system as a whole correctly describe interactions between components considered microscopically and macroscopically.

Coherent control of the vibrational state population in a nonpolar molecule

A coherent control scheme for the population distribution in the vibrational states of nonpolar molecules is proposed. Our theoretical analysis and results of numerical simulations for the interaction of the hydrogen molecular ion in its electronic ground state with an infrared laser pulse reveal a selective two-photon transition between the vibrational states via a coupling with the first excited dissociative state. We demonstrate that for a given temporal intensity profile the population transfer between vibrational states, or a superposition of vibrational states, can be made complete for a single chirped pulse or a train of chirped pulses, which accounts for the accumulated phase difference due to the ac Stark effect. Effects of a spatial intensity (or focal) averaging are discussed.

Fluorescence from the A2Σ+state suggests a non-Franck–Condon N2O+•vibrational state population after keV collisional activation with helium

A commercial analytical mass spectrometer coupled with a spectrograph and charge-coupled device (CCD) detector was used to obtain the collision-induced emission (CIE) spectra (190–1020 nm) from ion–molecule collisions between N2O+•and He at laboratory frame collision energies of 4–8 keV. The observed emissions were from the A2Σ+ → X2Π+system of N2O+•, excited fragments of the parent molecule, and the target gas. The relative emission intensities of the N2O+•ions and their fragments were independent of the ion translational energy within the 4–8 keV range, confirming the curve-crossing mechanism for translational-to-internal energy transfer in this system. Band emission intensities of [Formula: see text] and [Formula: see text] relative to the [Formula: see text] band within the N2O+•(A → X) system have been measured and compared with reference data. The results indicated that the collisionally excited ion has a decidedly non-Franck–Condon vibrational state distribution.

ChemInform Abstract: Vibrational Relaxation of Acetylene Produced by the Photolysis of Vinyl Bromide.

Vibrational relaxation of the ν2 and 2ν2 CC stretch vibrational states in the ground electronic state of acetylene has been investigated using infrared kinetic spectroscopy. Excited vibrational states of acetylene are produced by 193 nm ArF excimer laser photolysis of vinyl bromide in a room‐temperature helium buffer gas. Vibrational state populations are probed by direct absorption with the continuous single‐frequency output of a color center laser operating in the 3 μm region. The vibrational transitions observed were the ν2+ν3−ν2 and the 2ν2+ν3−2ν2 combination bands. Rate constants for He vibrational relaxation are 1.5(3)×10−14 and 2.1(7)×10−14 cm3 molecule−1 s−1 for ν2 and 2ν2, respectively.

Quasi-classical model of non-destructive wavepacket manipulation by intense ultrashort nonresonant laser pulses

A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 1013 W cm-2 are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typical of coherent control and femtochemistry. The 1sσ ground state of the D2 precursor is projected onto the available electronic states in D2+ (1sσg ground and 2pσu dissociative) and D++D+ (Coulomb explosion) by tunnel ionization by an ultrashort ‘pump’ pulse, and relative populations are found numerically. A generalized non-adiabatic treatment allows the dependence of the initial vibrational population distribution on laser intensity to be calculated. The wavepacket is approximated as a classical ensemble of particles moving on the 1sσg potential energy surface (PES), and hence follow trajectories of different amplitudes and frequencies depending on the initial vibrational state. The ‘control’ pulse introduces a time-dependent polarization of the molecular orbital, causing the PES to be modified according to the dynamic Stark effect and the transition dipole. The trajectories adjust in amplitude, frequency and phase-offset as work is done on or by the resulting force; comparing the perturbed and unperturbed trajectories allows the final vibrational state populations and phases to be determined. The action of the ‘probe’ pulse is represented by a discrete internuclear boundary, such that elements of the ensemble at a larger internuclear separation are assumed to be photodissociated. The vibrational populations predicted by the QCM are compared to recent quantum simulations (Niederhausen and Thumm 2008 Phys. Rev. A 77 013404), and a remarkable agreement has been found. The applicability of this model to femtosecond and attosecond time-scale experiments is discussed and the relation to established femtochemistry and coherent control techniques are explored.

Radio frequency magnetic field effects on molecular dynamics and iron uptake in cage proteins

The protein ferritin has a natural ferrihydrite nanoparticle that is superparamagnetic at room temperature. For native horse spleen ferritin, we measure the low field magnetic susceptibility of the nanoparticle as 2.2 x 10(-6) m(3) kg(-1) and its Néel relaxation time at about 10(-10) s. Superparamagnetic nanoparticles increase their internal energy when exposed to radio frequency magnetic fields due to the lag between magnetization and applied field. The energy is dissipated to the surrounding peptidic cage, altering the molecular dynamics and functioning of the protein. This leads to an increased population of low energy vibrational states under a magnetic field of 30 microT at 1 MHz, as measured via Raman spectroscopy. After 2 h of exposure, the proteins have a reduced iron intake rate of about 20%. Our results open a new path for the study of non-thermal bioeffects of radio frequency magnetic fields at the molecular scale.

The conformer resolved ultraviolet photodissociation of morpholine

Distinct sets of resolved photoproducts arising from the photodissociation of two neutral molecule conformers have been observed using H(Rydberg) atom photofragment translational spectroscopy methods. This study focuses on the photodissociation of morpholine, at many wavelengths between 250 nm and 193 nm. Morpholine, a six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O, exists as two ground state chair conformers that are distinguished by the orientation of the N–H bond (axial or equatorial) relative to the chair ring. Complementary MP2, QCISD and DFT calculations all predict the equatorial chair conformer to be lower in energy by ∼250 cm−1. CASPT2(8/8) calculations confirm that both conformers have similar electronic structures, and share a common first excited singlet state, arising as a result of electron promotion from the nitrogen lone pair orbital (n) to the 3s/σ* orbital centred on the N–H bond, forming a 1nσ* state. Photoexcitation to the 1nσ* potential energy surface (PES) leads to N–H bond extension, with the dissociating flux passing through a conical intersection with the ground state PES en route to morpholinyl radicals and H atom photoproducts. Careful analysis of the resulting H atom time-of-flight spectra reveals peaks attributable to the population of specific vibrational states of morpholinyl products arising from dissociation of both conformers, enabling determination of the energy difference between the two ground conformers of morpholine as 180 ± 50 cm−1, and the N–H bond strengths in both the axial and equatorial conformers as 33200 ± 50 cm−1 and 33380 ± 50 cm−1, respectively.

Microwave Control of Atomic Motion in Optical Lattices

We control the quantum mechanical motion of neutral atoms in an optical lattice by driving microwave transitions between spin states whose trapping potentials are spatially offset. Control of this offset with nanometer precision allows for adjustment of the coupling strength between different motional states, analogous to an adjustable effective Lamb-Dicke factor. This is used both for efficient one-dimensional sideband cooling of individual atoms to a vibrational ground state population of 97% and to drive coherent Rabi oscillation between arbitrary pairs of vibrational states. We further show that microwaves can drive well resolved transitions between motional states in maximally offset, shallow lattices, and thus in principle allow for coherent control of long-range quantum transport.

Standoff detection of nitrotoluenes using 213-nm amplified spontaneous emission from nitric oxide

A method of standoff detection based on the observation of laser-induced fluorescence-amplified spontaneous emission (LIF-ASE) is described. LIF-ASE generates uniaxial intensity distributions of the observed fluorescence with the majority of intensity propagating along the excitation axis in both the forward and backward directions. The detection of bulk vapor at significant standoff distances is readily achieved. This method was used to detect NO directly and as a photoproduct after 213-nm excitation of 2-, 3-, and 4-nitrotoluene. The NO LIF-ASE spectra were studied as a function of buffer gas. These studies showed that the emission from different vibrational states was dependent upon the buffer gas used, suggesting that the populations of vibrational states were influenced by the environment. A similar sensitivity of the vibrational populations was observed when the different nitroaromatic precursors were used in nitrogen buffer gas. Such sensitivity to environmental influences can be used to distinguish among the different nitroaromatic precursors and facilitate the identification of the bulk vapor of these analytes.

Temperature dependence of isotopic fractionation in CO 2 photolysis

The effect of temperature on the heavy isotope ratios of the products (CO and O 2) of photolysis of CO 2 by ∼185 nm photons was studied. The isotope ratios 18O/ 16O and 17O/ 16O in O 2 and CO and 13C/ 12C in CO increase with temperature by ∼1‰ per K. The increase in 17O/ 16O in O 2 is related linearly to the increase in 18O/ 16O with a slope 0.8 contrary to the mass dependent value of 0.5. Variations in factors related to the transition dipole moment and Boltzmann distribution of the vibrational state populations are probably responsible for the observed change. Temperature dependence of the gas production rate agrees with the reported cross-section variations.

Strong-field impulsive alignment in the presence of high temperatures and large centrifugal distortion

We achieve impulsive alignment in a $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ sample of diatomic iodine. This alignment results in an ensemble of coherent rotational wave packets with revival structures at 220 and $440\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The revivals contain many more oscillations than are typically seen in rotational wave packets. The high-temperature sample contains a very large distribution in $J$, with strong centrifugal distortions. We find that the high initial temperature, rather than the strength of the laser impulse, gives rise to the centrifugally distorted revival oscillations. We present polarization measurements of this rotational wave packet together with semiclassical simulations showing the importance of the thermal distribution of population in both rotational and vibrational states. Our results show that excitation of coherent rotations is possible even in a system where the thermal energy is large compared to the energy exchanged with the laser field.

Coherent control of population and pulse propagation beyond the rotating wave approximation

The dynamics of populations of the vibrational states in a NO molecule (one-photon absorption) and of the electronic states in a 4,4′-bis(dimethylamino) stilbene molecule (two-photon absorption) is studied versus the frequency, intensity and shape of the laser pulse. We show that specially designed infrared laser pulses can build selective populations of certain vibrational states. A detuning of light frequency from the vibrational resonance qualitatively changes the dynamics of populations. It is found out that the populations of the nonresonant levels follow the pulse shape adiabatically if the detuning significantly exceeds the inverse characteristic time of the change of the pulse. Depopulation of the nonresonant vibrational states leads to a high population of the resonant state at the end of the pulse. Complete breakdown of the standard rotating wave approximation for a two-photon absorption process is observed even for a rather small intensity of the laser pulse. An analytical solution for the interaction of a pulse with a three-level system beyond the rotating wave approximation is obtained, and it is in close agreement with the strict numerical solution of the amplitude equations. Special attention is paid to the population dynamics of randomly oriented molecules. The orientational disorder as well as the vibrations limit the coherent population transfer. Calculations show the strong role of the anisotropy of photoexcitation in the coherent control of populations which can affect the anisotropy of photobleaching.

Controlling dynamics in diatomic systems

Controlling molecular energetics using laser pulses is exemplified for nuclear motion in two different diatomic systems. The problem of finding the optimized field for maximizing a desired quantum dynamical target is formulated using an iterative method. The method is applied for two diatomic systems, HF and OH. The power spectra of the fields and evolution of populations of different vibrational states during transitions are obtained.

High-Resolution Vibration–Rotation Spectroscopy of CO2: Understanding the Boltzmann Distribution

In this undergraduate physical chemistry laboratory experiment, students acquire a high-resolution infrared absorption spectrum of carbon dioxide and use their data to show that the rotational�vibrational state populations follow a Boltzmann distribution. Data are acquired with a mid-infrared laser source and infrared detector. Appropriate absorption peak assignments are made by comparison of the experimental spectrum with the high-resolution transmission molecular absorption (HITRAN) database. Integrated absorption peak areas are used as a measure of relative rotational�vibrational state populations. The effect of temperature on the population distribution is explored by sampling the absorption spectrum at several different temperatures, giving students the opportunity to observe the properties of a Boltzmann distribution.

Dissociative electron attachment to HBr: A temperature effect

The effects of rovibrational temperature on dissociative electron attachment to hydrogen bromide has been investigated from the experimental and theoretical point of view. Theoretical calculations based on the nonlocal resonance model predict a strong temperature effect on the Br{sup -} fragment ion yield due to population of higher vibrational and rotational states. A crossed beam experimental setup consisting of a temperature controlled effusive molecular beam and a trochoidal electron monochromator has been used to confirm this prediction. The high degree of agreement between experiment and theory indicates the validity of the theoretical model and its underlying physical picture.

Mid-infrared vibrational spectra of discrete acetone-ligated cerium hydroxide cations

Cerium(iii) hydroxy reactive sites are responsible for several important heterogeneous catalysis processes, and understanding the reaction chemistry of substrate molecules like CO, H(2)O, and CH(3)OH as they occur in heterogeneous media is a challenging task. We report here the first infrared spectra of model gas-phase cerium complexes and use the results as a benchmark to assist evaluation of the accuracy of ab initio calculations. Complexes containing [CeOH](2+) ligated by three- and four-acetone molecules were generated by electrospray ionization and characterized using wavelength-selective infrared multiple photon dissociation (IRMPD). The C[double bond, length as m-dash]O stretching frequency for the [CeOH(acetone)(4)](2+) species appeared at 1650 cm(-1) and was red-shifted by 90 cm(-1) compared to unligated acetone. The magnitude of this shift for the carbonyl frequency was even greater for the [CeOH(acetone)(3)](2+) complex: the IRMPD peak consisted of two dissociation channels, an initial elimination of acetone at 1635 cm(-1), and elimination of acetone concurrent with a charge separation producing [CeO(acetone)](+) at 1599 cm(-1), with the overall frequency centered at 1616 cm(-1). The increasing red shift observed as the number of acetone ligands decreases from four to three is consistent with transfer of more electron density per ligand in the less coordinated complexes. The lower frequency measured for the elimination/charge separation process is likely due to a combination of: (a) anharmonicity resulting from population of higher vibrational states, and (b) absorption by the initially formed photofragment [CeOH(acetone)(2)](2+). The C-C stretching frequency in the complexes is also influenced by coordination to the metal: it is blue-shifted compared to bare acetone, indicating a slight strengthening of the C-C bond in the complex, with the intensity of the absorption decreasing with decreasing ligation. Density functional theory (DFT) calculations using three different functionals (VWN, B3LYP, and PBE0) were used to predict the infrared spectra of the complexes. Calculated frequencies for the carbonyl stretch are within 40 cm(-1) of the IRMPD of the three-acetone complex measured using the single acetone loss, and within 60 cm(-1) of the measurement for the four-acetone complexes. The B3LYP functionals provided the best agreement with the measured spectra, with the VWN modestly lower and PBE0 modestly higher. The C-C stretching frequencies calculated using B3LYP are higher in energy than the measured values by approximately 30 cm(-1), and reproduce the observed trend which shows that the C-C stretching frequency decreases with increasing ligation. Agreement between C-C frequency and calculation was not as good using the VWN functional, but still within 70 cm(-1). The results provide an evaluation of changes in the acceptor properties of the metal center as ligands are added, and of the utility of DFT for modeling f-block coordination complexes.