Overtone Transitions Research Articles

Action spectroscopy and temperature diagnostics of H3+ by chemical probing

Infrared absorption spectroscopy of few hundred H+(3) ions trapped in a 22-pole ion trap is presented using chemical probing as a sensitive detection technique down to the single ion level. By exciting selected overtone transitions of the (v(1)=0,v(2) (l)=3(1))<--(0,0(0)) vibrational band using an external cavity diode laser an accurate diagnostics measurement of the effective translational and rotational temperatures of the trapped ions was performed. The absolute accuracy of the measured transition frequencies was improved by a factor of four compared to previous plasma spectroscopy measurements using velocity modulation [Ventrudo et al., J. Chem. Phys. 100, 6263 (1994)]. The observed buffer gas cooling conditions in the ion trap indicate how to cool trapped H+(3) ions into the lowest ortho and para rotational states. Future experiments will utilize such an internally cold ion ensemble for state-selected dissociative recombination experiments at the heavy ion storage ring Test Storage Ring (TSR).

Anharmonic vibrational spectroscopy calculations for (NH3)(HF) and (NH3)(DF): fundamental, overtone, and combination transitions

In order to study the effects of hydrogen bonding on the spectroscopic properties of (NH3)(HF) and (NH3)(DF) complexes, vibrational spectra (including fundamental, overtone and combination transitions) were calculated using the vibrational self consistent field (VSCF) method. This ab initio VSCF method accounts for both one-dimensional anharmonicity and pair-wise mode-mode couplings for all vibrational modes of the molecule, using points on the potential energy surface (at the MP2/TZP level of theory in this study). An analysis of the coupling strength shows surprisingly important coupling effects from pair-wise interactions not expected to be major. This indicates the benefits of including all pair-wise mode-mode couplings for weakly bound systems. Hydrogen bonding induces approximately 20% red shifts for the HF and DF stretch frequencies. The corrections due to anharmonicity for these modes are -6% and -5%, respectively. The anharmonic corrections for the intermolecular stretch of (NH3)(HF) and (NH3)(DF) are each about -5%. The NH3 umbrella motion has virtually no anharmonic correction in the complex, whereas free ammonia experiences a -15% correction. Also, the closing motion as well as the opening motion is restricted. The 1 + 1 combination transition of the proton stretching and intermolecular stretching modes has remarkably large intensity, larger even than the intensities for the first overtone of the proton stretching modes. The anharmonic frequency for the fundamental HF stretch, 3268 cm(-1), is in good agreement with the experimental gas phase result, 3215 cm(-1). A comparison to solid rare-gas matrix data shows that the VSCF frequencies are a consistent improvement over the harmonic approximation. The experimental data also support the use of the MP2 level of theory for the associated electronic structure calculations.

Simulation study of solvent shifts of vibrational overtone spectra

Buckingham's theory of the solvent shift of vibrational spectral frequencies predicts that the shift of the v = 0 → n overtone transition is n times the shift of the fundamental v = 0 → 1. We test this prediction by molecular dynamics simulations using existing intermolecular potential models for liquid N2 and dilute N2 in liquid Ar, at standard state conditions. We extend Buckingham's theory by including additional intramolecular potential and perturbation terms which lead to solvent-induced anharmonicity, i.e. O(n 2) terms in the solvent shift. The simulations show that Buckingham's prediction is not accurate for N2 at standard liquid state conditions. We find that at these conditions there is a significant positive O(n 2) contribution to the solvent shifts and that for n ∼ 20 the shifts change sign from red to blue. Simulation results and indirect evidence from shock wave experiments with liquid N2 show that Buckingham's prediction is more accurate for high-pressure high-temperature conditions, where the shifts are blue and only slightly nonlinear in n.

Cascade lasing of molecular HBr in the four micron region pumped by a Nd:YAG laser

Due to a narrow window of high atmospheric transmission near 4 /spl mu/m there is a great deal of interest for a scalable laser energy source in this spectral region. We propose a concept combining the advantages of solid-state and gas laser technology. It takes advantage of a coincidence of a Nd:YAG laser line and an overtone transition of the molecule HBr. Tuning a Q-switched Nd:YAG laser to 1.3391 /spl mu/m allows us to excite the v(0 /spl rarr/ 3), J(4 /spl rarr/ 5) vibrational-rotational transition of HBr. To stabilize the pump frequency, a diode laser locked to this HBr transition seeds the Nd:YAG laser. Once excited, HBr can potentially lase in three subsequent steps to the ground state, two of which were observed experimentally, emitting up to three photons in the 4-/spl mu/m region. We present theoretical and experimental results demonstrating the operational principle of this laser system. The comparison of experiment and theory suggests: 1) that intracavity CO/sub 2/ can force the system to lase on only those transitions that are within the atmospheric transmission window, and 2) the existence of amplified spontaneous emission driven by pure rotational transitions.

Line-shape study of water vapour by tunable diode laser spectrometer in the 822?832�nm wavelength region

A near-infrared tunable diode laser absorption spectrometer is set up to measure the air-induced broadening coefficients and the line-strength parameters of water-vapour overtone transitions within the (2,1,1)←(0,0,0) band in the 822–832 nm wavelength region. A Hitachi HL8311 E double hetero-junction structure diode laser is used as a probe. The diode laser controller is home-built and stable within ±10 μA and ±10 mK, respectively. The laser-head mount has a simple design and provides easy access whenever changing of the laser head is required. The diode laser emission wavelength is thermally tuned between 50 °C and 12 °C. Thermal tuning of the diode laser emission wavelength is used to reveal the mode structure of the diode laser and to probe the overtone-band transitions of water vapour within its operating wavelength range. Current tuning of the diode laser is used at a fixed laser temperature to study the transitions one at a time. A balanced detector is used to improve the S/N ratio of the spectrum. A phase sensitive detection technique is followed to obtain the first-derivative spectra of the overtone transitions. The first-derivative spectra have been recorded at different air pressures inside the sample cell while the water-vapour pressure is kept fixed. The first-derivative spectrum is numerically integrated to obtain the original line shape. The original line shape is fitted with a Voigt profile by using a nonlinear least-squares fit program to extract the air-broadening coefficient and the line-strength parameter. The data obtained in our work is compared with the results of the HITRAN database.

Near infrared study of water-benzene mixtures at high temperatures and pressures.

Near-infrared absorption of water-benzene mixtures has been measured at temperatures and pressures in the ranges of 473-673 K and 100-400 bar, respectively. Concentrations of water and benzene in the water-rich phase of the mixtures were obtained from the integrated absorption intensities of the OH stretching overtone transition of water and the CH stretching overtone transition of benzene, respectively. Using these concentrations, the densities of the water-rich phase were estimated and compared with the average densities before mixing, which were calculated from literature densities of neat water and neat benzene. It is found that anomalously large volume expansion on the mixing occurs in the region enclosed by an extended line of the three-phase equilibrium curve and the one-phase critical curve of the mixtures, and the gas-liquid equilibrium curve of water. Furthermore, magnitude of the relative volume change increases with decreasing molar fraction of benzene in the present experimental range. It is suggested that dissolving a small amount of benzene in water induces a change in the fluid density from a liquidlike condition to a gaslike condition in the vicinity of the critical region.

New observations of weak overtone transitions of N 2O by ICLAS-VeCSEL near 1.07 μm

We have completed our investigations of the weak overtone transitions of nitrous oxide near 1.05 μm by the study of the 9090–9560 cm −1 region by intracavity laser absorption spectroscopy (ICLAS) using a vertical external cavity surface emitting laser (VeCSEL). Eight bands were rotationally analyzed. Two hot bands are newly observed while the rotational analysis of the six cold bands has been improved. This Note includes the correction of a few misprints in the rovibrational parameters and the reconsideration of the analysis of an interpolyad Coriolis interaction system given in our previous contribution [J. Mol. Spectrosc. 220 (2003) 80].

Saturation-dip measurements in the 2ν2 overtone band of OCS with a CO2-laser/microwave-sideband spectrometer

We have employed a CO2-laser/microwave-sideband spectrometer to carry out saturation-dip sub-Doppler measurements of a number of 2ν2 overtone transitions of OCS in the 10-μm region. The OCS frequencies have been obtained with an absolute accuracy of order ±37 kHz, as determined from a careful analysis of the combined uncertainties in the frequency of our Lamb-dip-locked laser and the centers of the observed OCS saturation signals. Our ±37 kHz measurement accuracy is consistent with literature OCS sub-Doppler data obtained on a similar instrument. The results serve to extend the comb of precise reference frequencies in the 10-μm region and to determine the magnitudes of systematic and random uncertainties of our CO2 laser.

Spectral condensation near molecular transitions in intracavity laser spectroscopy with vertical external cavity surface emitting lasers

Vertical external cavity surface emitting lasers (VeCSELs) are ideal amplification media for intracavity laser absorption spectroscopy (ICLAS) around 1 μm. However, we noted that, in some spectra recorded by ICLAS-VeCSEL, the observed line profiles of very weak overtone transitions were clearly asymmetric, this distortion being more pronounced for the strongest lines. More spectacular effects were observed when we investigated the acetylene spectrum: while the weakest absorption lines appeared as dips superimposed on the baseline as usual in ICLAS, the strongest lines mostly disappeared and were replaced by emission lines slightly blue shifted from the absorption line center. This effect, called “spectral condensation”, was, in fact, discovered near atomic transitions, more than 30 years ago, using pulsed lasers. Using acetylene and water as absorbers, we have systematically studied the different factors of importance for spectral condensation and showed that it increases with the pumping rate, the generation time and the line intensity. Spectral condensation was also found to increase with the pressure of the intracavity gas sample up to a value of a few Torrs, and to decrease afterwards. The appearance of spectral condensation has been shown to be highly correlated to time oscillations of the total laser power. Of importance for intensity measurements, this systematic study has showed that condensation effects may appear for long generation times even at low pumping rate and even with extremely weak absorption lines. A discussion of the results in relation with different interpretations available in the literature is presented with a particular emphasis on the theory based on the coherent interaction of electro-magnetic field with optically dense resonant extended medium leading to a parametrical amplification without population inversion.

Infrared intensities of furan, pyrrole and thiophene: beyond the double harmonic approximation

Infrared intensities of the fundamental, overtone and combination transitions in furan, pyrrole and thiophene have been calculated using the variational normal coordinate code MULTIMODE. We use pure vibrational wavefunctions, and quartic force fields and cubic dipole moment vector surfaces, generated by density functional theory. The results are compared graphically with second-order perturbation calculations and with relative intensities from experiment for furan and pyrrole.

Overtone spectroscopy in the blue region by cavity-enhanced absorption spectroscopy with a mode-locked femtosecond laser: application to acetylene

The wide spectral coverage and high sensitivity of mode-locked cavity enhanced absorption spectroscopy (ML-CEAS) are illustrated by the observation of a very high overtone transition of C 2H 2 in the blue spectral region (420 nm, 8 quanta of CH stretch excitation), which was easily accessed by frequency-doubling a mode-locked femtosecond Ti:Sa laser. The detection limit is about 10 −8/cm. The rotational analysis of this Σ u +–Σ g + parallel band, centred at 23 813.244 cm −1, is presented and the vibrational assignment is discussed.

Near-infrared spectroscopic study of water at high temperatures and pressures

Near-infrared absorption of the OH stretching overtone transition of water has been measured at temperatures and pressures in the ranges of 373–673 K and 20–400 bar, respectively. The absorption profile at 673 K and 400 bar retains a mark of rotational structure, indicating that an appreciable proportion of water molecules can rotate quite freely. The molar absorption intensity decreases linearly with increasing pressure in the low-pressure region. Enthalpy for dimerization has been estimated to be 15±3 kJ/mol from the temperature dependence of the slopes. Plots of the molar absorption intensity against molar concentration are observed to be located on a single curve irrespective of the temperature. This fact indicates that the ratio of hydrogen-bond formation is largely dependent on the molar density only. A good correlation between the molar absorption intensity and the first moments of the band has been found out; this will be useful in the study of aqueous mixtures.

Quantum property of solid hydrogen as revealed by high-resolution laser spectroscopy

Pure vibrational overtone transitions of solid parahydrogen are studied using high-resolution laser spectroscopy. Extremely narrow spectral linewidth (∼20 MHz) allows us to observe rich spectral structure that originates in subtle intermolecular interactions in the crystal. It is found that anisotropy of the distribution of zero-point lattice vibration of hydrogen molecules perturbs the energy levels of the vibrationally excited states significantly. A large amplitude of zero-point lattice vibration, an intrinsic propoerty of quantum solids, is directly observed from the present high-resolution spectroscopy. The first observation of a pure vibrational overtone transition of solid orthodeuterium is also discussed.

Molecular-beam/surface-science apparatus for state-resolved chemisorption studies using pulsed-laser preparation

We describe a new apparatus that combines pulsed laser excitation in a molecular beam with surface-science methods for preparation of clean single-crystal surfaces and detection of adsorbates to enable state-selected studies of gas–surface reaction dynamics. Reactant molecules are prepared in specific vibrationally excited states via overtone pumping using tunable, narrow-band laser radiation. The collision-free environment of the molecular beam prevents relaxation of the prepared molecules before impact on the target surface and enables complete control over the collision energy and incidence angle. Chemisorption products are detected after a given deposition time by Auger electron spectroscopy. To achieve sufficient beam flux of state-selected reactant molecules for product detection by standard surface-science techniques, we use a high-intensity, short-pulse molecular-beam source matched to the low duty cycle of the pulsed lasers used in our experiments. We present the design and characterization of this new apparatus together with a scheme for generating infrared laser pulses of high spectral brightness for saturating weak vibrational overtone transitions within a significant volume of the molecular beam. The effectiveness of our approach is demonstrated by state-resolved sticking coefficient measurements for overtone-excited (2ν3) CH4 on Ni(100) for a range of impact energies.

Overtone transitions of diatomic dimers XH−-XH− in ionic crystals

The resonance interaction of molecular defects XH– in ionic crystals is considered. In the dipole approximation, the frequencies and intensities of vibrational transitions of diatomic dimers mXH−-nXH− are found for the exact (m=n) and quasi-exact (m≠n) resonances. The interaction-atrix elements are obtained with regard to the mechanical anharmonicity in the second order of the theory of perturbation for the case of a linear dependence of the dipole moment of a diatomic molecule on the vibrational coordinate. The expressions obtained are applied to the calculation of overtones in the absorption spectrum of dimers mSH−-nSH− in a KCl crystal.

Weak overtone transitions of N 2O around 1.05 μm by ICLAS-VECSEL

The weak overtone transitions of nitrous oxide, N 2O, have been investigated around 1.05 μm by Fourier Transform spectroscopy and intracavity laser absorption spectroscopy (ICLAS) using different vertical external cavity surface emitting lasers (VECSELs). Nine bands were rotationally analyzed revealing the occurrence of some local rotational perturbations. The interaction mechanisms and the perturbers are univocally assigned on the basis of the effective Hamiltonian model. In two cases, interpolyad couplings were evidenced indicating that the polyad version of the effective Hamiltonian has to be extended to include Coriolis and interpolyad anharmonic interactions.

Counterintuitive multiphoton pulse sequences in molecular isomerization. I. Selectivity and robustness of competing multiphoton stimulated Raman adiabatic passage processes

We use a 28-level system consisting of the vibrational bend states of the HCN/HNC system as a model system for the investigation of population transfer across the isomerization barrier using counterintuitively ordered multiphoton (MP) laser pulses. This (m+n)-STIRAP (stimulated Raman-adiabatic passage) arrangement with m pump- and n Stokes-photons is found to be capable of driving HCN→HNC transition states selectively and in a robust manner, despite the presence of near-degenerate parallel processes of different photonicities. Employing Gaussian pulses with widths of 80 ps, the process of highest photonicity we demonstrate is (4+6)-STIRAP, and we investigate systematically the robustness against variations in pulse amplitudes and frequencies for all parallel processes with m=2 and 3, and n=1–6. We discuss the conditions under which selective population transfer is obtained. We also compare with (1+1)-STIRAP using overtone transitions to drive the same state-specific processes and conclude that the few-step sequential MP-STIRAP isomerization is a feasible alternative to the sequential (1+1)-STIRAP based method proposed by Kurkal and Rice [J. Phys. Chem. B 105, 6488 (2001)].

Vibrational Overtone Activated Photo-Cross-Linking of Ethylene Glycol Dimethacrylate Using Benzoyl Peroxide and 2,2‘-(Azobis)isobutyronitrile as Initiators

Laser vibrational overtone initiated polymerization of ethylene glycol dimethacrylate (EGDMA) has been demonstrated using two initiators, benzoyl peroxide (BP) and 2,2‘-(azobis)isobutyronitrile (AIBN). Excitation of the initiator occurs through absorption of a single photon by the fifth CH stretching vibrational overtone transition at 604 nm (16 556 cm-1) for BP and 628 nm (15 924 cm-1) for AIBN. This vibrational excitation creates radicals that initiate radical chain polymerization. Monomer conversion was followed quantitatively by monitoring the intensity change of the vinyl combination band at 6169 cm-1 in the first overtone spectrum of EGDMA. Considering the concentration, the molar extinction coefficient, and laser power, AIBN was a factor of 3.2 more efficient than BP for vibrational overtone polymerization. In both cases, the rate of polymerization depends on the excitation wavelength. The influence of the initiator concentration and the photolysis duration is also presented.

Ab initio calculation of anharmonic vibrational spectra of hydrazine and hydrazine-d 4

Anharmonic vibrational spectra of gauche conformers of hydrazine and hydrazine-d 4 were studied using vibrational self-consistent field (VSCF) and correlation-corrected vibrational self-consistent field (CC-VSCF) methods within second order Møller–Plesset perturbation theory (MP2). Wavenumbers and intensities of both fundamental and overtone transitions were computed in TZV(d,p) and TZV(2d,2p) basis sets. Anharmonic spectra calculated on MP2/TZV(2d,2p) potential energy surface are in much better agreement with the experiment than those obtained in TZV(d,p) basis, but the energy sequence of ν(N–H) stretching modes is preserved in both basis sets. CC-VSCF computed wavenumber of the torsional mode overestimates its experimental position by 80 and 34 cm −1 in TZV(d,p) and TZV(2d,2p) basis sets, respectively. The anharmonic spectrum of hydrazine-d 4 computed at the same level of theory also provides adequate description of the experimental data, with the position of torsional vibration 9 cm −1 above the observed one.

Sensitivity of molecular vibrational dynamics to energy exchange rateconstants

The sensitivity of molecular vibrational population dynamics, governing the COlaser operated in fundamental and overtone transitions, to vibration-to-vibrationrate constants is investigated. With this aim, three rate constant sets have beenused, differing in their completeness (i.e. accounting for single-quantumexchange only, or for multi-quantum exchange with a limited number of rateconstants obtained by semiclassical calculations, and, finally, with anexhaustive set of rate constants including asymmetric exchange processes,as well) and in the employed interaction potential. The most completeset among these three is introduced in this paper. An existing earlierkinetic model was updated to include the latter new data. Comparison ofdata produced by kinetic modelling with the above mentioned sets ofrate constants shows that the vibrational distribution function, and, inparticular, the CO overtone laser characteristics, are very sensitive to thechoice of the model. The most complete model predicts slower evolution ofthe vibrational distribution, in qualitative agreement with experiments.