Multiple photon excitation of polyatomic molecules from the many rotational states by an intense pulse of IR radiation

The investigation of the role of pulse duration in the absorption of CO₂ laser pulses by polyatomic molecules has shown that the rotational relaxation does not contribute significantly to the absorption of intense IR radiation. This proves the existence of purely radiative depletion of all rotational states in monochromatic fields of moderate intensity.     

Efficient rotational Raman conversion of XeCl laser radiation in H2

2 is investigated with a XeCl laser. It is shown that energy conversion to rotational Stokes radiation can be efficiently obtained by properly choosing the focusing geometry of pump radiation and the pressure of the Raman medium even at moderate pulse energies. Energy conversion to the first rotational Stokes at 313.8 nm with efficiencies as high as 38% is obtained with a circularly polarised XeCl pump beam of 10-mJ energy focused in 30 bar of H₂. The spectral and optical characteristics of the pump and the rotational Stokes radiation are analysed. Received: 29 May 1996/Revised version: 7 March 1997     

Determination of the rotational temperature and the molecular density in an expanding NH3 jet by infrared absorption

Rotational temperatures are determined by measuring the absorption of infrared laser radiation. The possibilities of this method are critically examined and tested. As a result the molecular density in the expansion could be determined, too. Color-center laser radiation has been absorbed by a molecular jet of NH₃. An anomalous line shape has been observed, related to a Doppler shift from molecules moving along the various streamlines. No deviations from a thermal rotational distribution have been observed.Work supported by Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)     

Hamiltonians with quartic anharmonicities: Comparison with EPR measurements in SrTiO3

Rigorous bounds have been found for Hamiltonians with quartic anharmonicity in the single particle potential. They reveal that EPR measurements in SrTiO₃ can only be explained by use of those Hamiltonians alone, when there exists a strong cubic anisotropy in the anharmonicity.     

Four-photon polarization spectroscopy of the rotational resonances in liquids in the range 0–3 THz

The spectra of the coherent molecular rotation that coincide with the rotational spectra of the corresponding molecules in the gas phase are measured for the first time using four-photon coherent laser spectroscopy in the range 0–100 cm^?1 in several liquids (CCl₄, H₂O₂, D₂O, and H₂O). The measured spectra make it possible to separate the spectral contributions of the slow rotational molecular motions about the equilibrium and the fast rotations. The selectivity of the action of the microwave radiation on biological objects can be increased using the results obtained.     

UV laser desorption of nitric oxide from semiconducting C<Subscript>60</Subscript>/Cu(111)

The desorption of NO from a well-characterized, epitaxially grown semiconducting C₆₀ surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ₁=7.0×10^-17 cm² and σ₂=5.5×10^-17 cm² for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the -bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈E_kin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of T_rot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of T_rot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated. PACS 42.62.Fi; 34.50.Dy; 68.49.Df; 68.43.Tj; 79.20.La     

Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value P _cr; (ii) the degree of excitation abruptly increases after crossing the boundary where P = P _cr; (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > P _cr, number 00000 is determined only by energy density Φ = Pτ_P, where τ_P is the laser pulse duration, 00000 monotonically increases with increasing Φ. The model is in good agreement with the experimental data.     

Dynamics of a Loss-Modulated, Two-Wave CO2 Laser Lasing on Lines of Different Vibrational Bands

The nonlinear dynamics of a loss-modulated, two-wave CO₂ laser lasing on vibrational–rotational lines of different vibrational bands has been investigated theoretically and experimentally. It is shown that the time and energy parameters of laser radiation can be controlled within fairly wide limits by changing the depth and frequency of modulation and the ratio between the constant components of the losses.     

Selective Alignment of D2 Induced by Two Ultrashort Laser Pulses

通过求解D₂分子在飞秒激光场中的含时薛定谔方程,研究了室温下D₂分子在超快飞秒激光驱动下的的转动波包动力学. 选择用第一束超短飞秒脉冲与温度为300 K的D₂分子系综相互作用产生一个相干转动波包,用第二束超短飞秒脉冲在波包的1/4和3/4恢复周期选择操纵D₂分子取向. 研究结果表明,通过选择两束超短飞秒脉冲的延迟时间,可以有效控制D₂分子转动波包中奇偶态的相对布居,从而选择性的控制D₂分子取向.     

Interaction of intense beams of highly vibrationally excited molecules with molecules (clusters) condensed on a cold surface

The interaction of intense beams of SF₆ and CF₃I molecules, excited by powerful IR laser radiation to high vibrational states (0.3 eV ≤ E _vib ≤ 2.0 eV), with molecules (clusters) condensed on a cold surface (T _s ≈ 80–85 K) has been studied. The probability that the excited and unexcited molecules are reflected from the cold metal surface covered by condensed molecules (clusters), as well as the probability that such excited and unexcited molecules are transmitted through a cooled multichannel metal plate and a converging cone oriented at an angle relative to the molecular beam axis, has been determined. Expressions for these probabilities of reflection and transmission as functions of the angle of incidence and the parameters of the exciting laser radiation and the molecular beam are obtained. It is shown that highly vibrationally excited molecules are reflected from the surface and transmitted through the plates and cones with a much higher probability than unexcited molecules. The results suggest that this phenomenon can be used for the separation of molecules in a beam with respect to isotope (or atomic) composition.     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

基于光放大的长脉冲抽运太赫兹激光

为了获得较窄增益带宽、较大光强的太赫兹辐射,尝试了利用TEA CO₂激光器对6 W可调谐连续波CO₂种子激光实行双程光放大实验,以期望获得长脉冲、高峰值功率的抽运光.通过实验,获得了大约30倍的放大系数,放大光输出功率随种子光注入功率的增加而增加,其中10 P （20）支线表现更加突出,并且被放大的激光支线半峰全宽在14 μs左右.基于这种长脉冲抽运源,建立了一种产生脉冲太赫兹的动力学模型,给出了产生太赫兹辐射的饱和抽运光强表达式,确定了抽运光的光强范围.另外, 关键词： 激光物理 长脉冲 动力学模型 太赫兹激光     

Quantitative study of EγEγ correlations in the 122Sn + 106 MeV 12C compound system

The E_γE_γ correlations in the ¹²²Sn + 106 MeV ¹²C compound system have been measured by using the techniques of time of flight to reject events due to neutrons and of unfolding to remove events due to Compton escape. The rotational correlations in the measured matrix can be followed up to E_γ = 1.2 MeV which is shown to correspond to a collective spin of 15. An attempt to estimate the amount of aligned angular momentum by utilizing the intensities in the unfolded matrix failed mainly because at least half of all E2 γ-rays showed no rotational correlations. An independent evidence for this background of uncorrelated events is obtained by comparing the rotational correlations in the data with the rotational correlations obtained by simulating the decay of the known discrete levels in ^{127, 128}Ba on a computer. The comparison indicates that the observed rotational correlations are mainly due to the known discrete transitions. While the moment of inertia obtained from the correlation pattern is less than 80 % of the rigid-body value the high-energy end of the spectrum of E2 radiation can only be explained if γ-ray energy and spin are related by the rigid-body moment of inertia. The area of positive correlations that occurs in most published correlation plots at E₁ = E₂ and at the high-energy end of the E2 radiation is understood from the unfolding to be an artifact of the subtraction procedure.     

Optimization of a semiconductor lidar for detecting atmospheric molecular iodine and hydrogen

A lidar equation for Raman backscattering of semiconductor laser radiation by I₂ and H₂ molecules is solved. The obtained results allow laser wavelength optimization for probing atmospheric I₂ and H₂ molecules at a distance up to 2 km.     

Polarization labeling spectroscopy of NO2

Polarization-labeling spectroscopy is used to simplify the visible spectrum of nitrogen dioxide. Nonlinear polarization effects are used to label and probe molecules in selected rotational-vibrational levels. Those labeled molecules can be detected by their depolarizing effect on a probe laser. The resulting spectra are considerably less complicated than normal absorption spectra. P- and R-branch transitions can be readily identified, and approximate excited-state rotational constants are determined. With more powerful lasers, it may be possible to use polarization labeling to unravel the very complicated vibrational structure of the NO₂ excited states.     

Estimation of rotational and vibrational constants for diatomic molecules using Mattera's potential function

Summary The potential function suggested by Mattera, Salvo, Terreni and Tommasini is investigated further and the expressions for rotational constant α_e and anharmonicity constantw _e x _e are obtained. The α_e andw _e x _e values of twentyseven diatomic molecules are calculated using those expressions and they are compared with their experimental values.     

Diode laser study of IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF6 molecules cooled in a pulsed free jet

A pulsed frequency tunable diode laser was used to investigate the IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF₆ molecules cooled in a pulsed free jet at exciting energy densities between ≈10^-2 and 2.3 J cm^-2. The depletion of all rotational sublevels was effective at considerable (≈5–11 cm^-1) pumping frequency detunings from the linear absorption spectrum (LAS) of the molecule the width of which under the conditions of experiment (T_rot ≈ 18 K) was ≈2–3 cm^-1. The fraction of molecules excited by a pumping pulse from individual rotational sublevels was measured and its dependence on the exciting pulse frequency and energy density investigated. The effect of collisions on the depletion of the rotational sublevels was studied.     

Kinetic mechanisms of initiating hydrogen-oxygen mixture combustion through the excitation of electronic degrees of freedom of molecular oxygen by laser radiation

Kinetic mechanisms resulting in the enhancement of combustion of H₂+O₂ mixtures when O₂ molecules are excited to the a ¹Δ_g and b ¹Σ _g ⁺ states with laser radiation (λ=1.268 and 0.762 μm) are analyzed. It is shown that the excitation of O₂ molecules by the laser radiation leads to the appearance of new O, H, and OH formation channels; promotes the ignition of the starting mixture; and reduces the self-ignition temperature. With initial pressures in the range P ₀=10³–10⁴ Pa, the self-ignition temperature can be reduced to 300 K even at relatively low energies of the laser radiation with λ=0.762 μm.     

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 m in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D₂ from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a magnifying glass.Heisenberg fellow of the Deutsche Forschungsgemeinschaft