IR multiphoton absorption of SF6 in flow with Ar at moderate energy fluences

6 in flow with Ar (SF₆: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (T_V≃230 K, T_R≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm², which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO₂-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF₆ MPA were studied for several CO₂-laser lines which are in a good resonance with the linear absorption spectrum of the ν₃ vibration of SF₆ at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF₆ with Ar was also studied. The results are compared with those obtained in earlier studies. Received: 4 September 1995/Revised version: 15 February 1996     

Disintegration of argon clusters in collisions with highly vibrationally excited SF6 molecules in crossed molecular and cluster beams

It has been found that collisions of highly vibrationally excited SF₆ molecules (with the vibrational energy E _vib ≥ 0.5–2.0 eV) with Ar _N clusters (where N ≤ 30–40 is the number of atoms in a cluster) in crossed molecular and cluster beams result in capture of molecules followed by complete disintegration of the clusters. Possible applications of the effect for selective doping of clusters with molecules, laser separation of isotopes, and selective transport of molecules to the surface are discussed.     

Multiphoton absorption of broad-band CO2 laser radiation by SF6

Increase of the emission bandwidth of a high-pressure CO₂ laser up to 1.5 cm^–1 increases the multiphoton absorption cross-section of SF₆. Comparison with the previously found [9] increased absorption for shorter pulses suggests that this is also a bandwidth effect. Spectral structures as narrow as 1 cm^–1 above the 10^th absorption step are invoked to explain the observations. The temperature effect, which disappears in the broad-band case, confirms this view.     

Spectral and energetic characteristics of passing vibrationally excited SF6 molecules through a cooled multichannel plate

We study the spectral and energetic characteristics of SF₆ molecules excited in a pulsed molecular beam by intense IR laser radiation into high-lying vibrational states (with the energy E _v ≥ 0.3–2.0 eV) that pass through a multichannel metal plate that is cooled to T _s ? 80–85 K and is inclined to the beam axis. From comparison of the measured characteristics with the spectral and energetic characteristics of the IR multiphoton absorption of these molecules, we find that they correlate rather well with each other. The results obtained allow us to conclude that, due to different probabilities of passing vibrationally excited and unexcited molecules through the multichannel plate, it is possible to separate molecules in the beam with respect to their isotopic (component) composition.     

Collision relaxation cross section of highly vibrationally excited molecules

Through quantum-beat spectroscopy collision relaxation of a high vibrational level of SO2 at 44 877.52 cm(-1) is characterized. This is a first measurement of collision relaxation for a single, highly excited vibrational level. The deduced relaxation cross section of this excited level by Ar is 216 A(2), 5 times the area of the hard sphere, and by an ambient temperature SO2 molecule is 969 A(2), almost 20 times the hard sphere. These cross sections indicate that relaxation collisions of highly vibrationally excited molecules have effective distances much longer than van der Waals radii and involve mechanisms qualitatively different from lower excitations.     

Symmetry-resolved absorption spectra of vibrationally excited CO2 molecules

Symmetry-resolved x-ray absorption spectroscopy has been first carried out on high-temperature molecules. From the angle-resolved ion yield spectra of CO2 both at room temperature and at 430 degrees C, symmetry-resolved absorption profiles of the C 1s(-1) 2pi(u) and O 1s(-1) 2pi(u) resonances have been extracted for the vibrational ground state molecules and bending-vibration excited ones. The profiles change dramatically between them, and the Renner-Teller effect becomes more evident for the vibrationally excited molecules. The effects of the multimode vibronic coupling are suggested for the O 1s(-1) 2pi(u) and O 1s(-1) 3s sigma(g) resonances.     

Disintegration of Ar N,Kr N,and (N2) N clusters during collisions with highly vibrationally excited SF6 molecules

It is shown that, when highly vibrationally excited SF₆ molecules (with vibrational energy E _vib ≥ 0.5–2 eV) collide with weakly bound van der Waals Ar _N , Kr _N , or (N₂) _N (N ≤ 30–40 atoms in a cluster) clusters in intersecting molecular and cluster beams, the molecules are trapped by the clusters, the clusters then undergo full disintegration, and the trapped molecules become free. The method of studying this process and the results obtained are described. The possibilities of application of this method for selective doping of clusters by molecules, laser separation of isotopes, and selective transportation of molecules to a surface are discussed.     

A differential technique to measure small product yield increments in a time-of-flight mass spectrometer for investigating IR spectra of highly excited polyatomics

We present an experimental technique for measuring small product yield increments in a specially designed time-of-flight mass spectrometer with a supersonic jet. Advantages of this technique are demonstrated in measuring the photodissociation IR spectrum (990–1050 cm^–1) of CF₃I molecules with vibrational energy at the threshold value. The differential technique has enabled us to reveal an anomalously narrow width of the spike of thev ₁ band of CF₃I. We discuss this anomaly.     

The characteristics of multiple-photon absorption of SF6 molecules cooled in free jet expansion from a pulsed supersonic nozzle

The multiple-photon absorption of pulsed TEA CO₂ laser radiation by SF₆ molecules cooled toT _R40K andT _v160K in the free jet expansion from a pulsed supersonic nozzle has been investigated at energy fluences of 0.1 to 3.0 J·cm^–2.For practically all laser lines which coincide with the linear absorption spectrum of thev ₃ vibrational mode of SF₆ [P(12)...P(28), 10,6 m], the dependence of the absorbed energyE _ab on the exciting energy fluence was found to be steeper than linearE _abⁿ, wheren=(1.1 to 1.8). Considerable increases of the absorption cross sections with increasing energy fluence were observed.The fraction of the molecules interacting with the laser radiation is estimated.     

Collisional effects in IR multiphoton absorption of CDF3

Energy absorption by CDF₃ for different energy fluences from an intense TEA CO₂ laser is reported as function of the pressure of CDF₃ and various bath gases (He, Ne, Ar, Xe, and CHF₃). Collisional behavior at different conditions is discussed.     

Evolution of the homogeneous IR spectrum of a highly vibrationally excited molecule as a result of a change in its vibrational energy

Using a photodissociation technique, we have measured the IR spectrum of thev ₂₁ mode of the (CF₃)₃ CI molecule with a vibrational energy ofE ₂=42500±3500 cm^–1 which is more than two times the dissociation energy. The experimental spectrum of a Lorenzian shape with a halfwidth of ₂=10.8±1.5 cm^-1 has been analyzed simultaneously with the results of the preceding work (₁=8.6±0.6 cm^-1) that were obtained at a lower vibrational energy (E ₁=36500±2500 cm^–1).     

高振动激发LiCs与CO_2碰撞中的振动-转动能量转移

受激发射泵浦得到的LiCs高位振动态与CO_2碰撞,研究了LiCs不同能量对碰撞能量转移过程的影响.利用高分辨率瞬时激光感应荧光(LIF)测量得到CO_2(0000,J=2-74)原生转动态分布.转动布居数的半对数描绘给出了一个双指数结构,得到了一个T_a=660±73K[对LiCs(E=4300cm~(-1))]和550±61K[对LiCs(E=3700cm~(-1))]的低能分布,一个T_b=2380±261K[对LiCs(E=4300cm~(-1))]和1980±217K[对LiCs(E=3700cm~(-1))]的高能分布,低能分布属于弹性或弱非弹性碰撞,高能分布属于强非弹性碰撞.转动分布对LiCs能量是敏感的.但弹性与非弹性分支比基本是相同的.在一次碰撞的条件下,测量了各J态的出现和倒空速率系数,它们仅弱依赖于LiCs激发能.出现和倒空速率系数对不同LiCs能量基本一致,是振动-转动/平移弛豫轨道的重要证据.     

Computer simulation of multiphoton excitation of SF6 molecules cooled by pulsed supersonic expansion

The initial ground-state dependence of the multiphoton excitation of SF₆ is numerically estimated using two different approximations for thev ₃ vibration-rotation levels and for the laser line width. From this calculations we have identified and explained some features seen in recent experimental data.     

CARS spectra of thermally excited SF6 molecules

CARS spectra of thev ₁ mode of thermally excited SF₆ were calculated numerically. The influence of the vibrational quasicontinuum on the CARS spectra has been considered by introducing different types of the homogeneous broadening at different vibrational levels. The appearance of additional lines in the CARS spectrum due to mixing of high-lying vibrational levels by Fermi coupling was considered numerically in the frame of a simple model. A comparison of calculated and experimental spectra has been made.     

Direct measurement of multiphoton molecular absorption of IR laser radiation by pyroelectric detector

The applicability of a pyroelectric detector (PED) to multiphoton absorption measurements (MPA) under ir laser excitation of molecules was examined. The possibility of MPA measurement of SF₆ molecules in the pressure range of 10⁻³÷10⁻¹ Torr with a time resolution of ∼ 7 μs has been shown. It has been demonstrated that the degree of vibrational relaxation of highly vibrationally excited molecules at PED surface does not depend on excitation level under the studied conditions.     

Relaxation dynamics of vibrationally highly excited H2 molecules on a Cu surface

Relaxation dynamics of vibrationally highly excited H₂ molecules on a Cu surface has been calculated by using an highly accurate potential energy surface. The results show an increase of dissociation probability as a function of vibrational quantum level and a practical absence of vibrational deactivation at low kinetic energies of impinging molecules.     

Quantum computation with vibrationally excited molecules

A new physical implementation for quantum computation is proposed. The vibrational modes of molecules are used to encode qubit systems. Global quantum logic gates are realized using shaped femtosecond laser pulses which are calculated applying optimal control theory. The scaling of the system is favorable; sources for decoherence can be eliminated. A complete set of one- and two-quantum gates is presented for a specific molecule. Detailed analysis regarding experimental realization shows that the structural resolution of today's pulse shapers is easily sufficient for pulse formation.     

Picosecond dynamics of IR reflection and transmission by highly excited semiconductors

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 3, pp. 438–444, 1990.     

Improved separation of the rare sulfur isotopes by infrared multiphoton dissociation of SF6

The dissociation probabilities of³²SF₆ and some of³⁴SF₆ have been measured at a large number of CO₂ laser lines both at room temperature and at 140 K. The longwavelength wing of this dissociation spectrum is exponential in the wavenumber. Its logarithmic slope is proportional to the inverse temperature. Selectivities are high enough at 140 K, that the photons are consumed only for the rare isotope in the case of³⁴SF₆ and nearly so for³⁶SF₆. For³³SF₆ further improvement of the selectivity would be desirable.