Relative photodissociation cross-section of NaCs molecule,using argon ion laser lines

Laser-induced photodissociation of NaCs molecule has been observed when a mixture of Na and Cs metal vapour in a glass cell was irradiated by most of the lines of an argon ion laser. The photodissociation results in the 3P state of Na atoms which is correlated with theF ¹Σ⁺ and G¹π molecular states of NaCs. Distribution of photofragments over fine structure components 3² P _3/2 and 3² P _1/2 of Na has been studied. The ratio of intensity ofD ₂ line (5890 Å) toD ₁ line (5896 Å) of Na varies from around 2 at 5145 Å to about 3.5 at 4579 Å. The relative photodissociation cross-section increases monotonically as the wave-length of laser light decreases from 5145 Å to 4579 Å. It is seen that the 4579 Å photon is about 200 times more effective than the 5145 Å photon in causing the photoreaction NaCs + (Ar⁺ photon) → Na*(3P) + Cs(6S).     

Determination of photodissociation anisotropy parameter β for RbI molecules by sub-Doppler spectroscopy

The photodissociation of RbI molecules upon exposure to polarized laser radiation with a wave-length of 266 nm is investigated. The anisotropy parameter β and the parameter ζ characterizing thermal motion of RbI molecules are determined by analyzing the Doppler absorption profiles of Rb atoms (5² S ^1/2) formed through photodissociation of RbI molecules. The systematic error in determination of the parameter β due to the layer thickness is examined.     

Using ion imaging to analyze higher-order moments of the angular distributions of photofragments

We investigate the effect of the hexadecapole (K=4) polarization moment on the spatial distributions of angular momenta for atoms produced during the photodissociation of diatomic or triatomic molecules by polarized radiation. We derive general expressions for the angular distributions of the atomic density matrix for K = 2, 4 and expressions for the corresponding anisotropy parameters that contain all information on the photodissociation dynamics. We show that these anisotropy parameters can be experimentally determined by using ion imaging. We consider oxygen atoms in the ¹ D ₂ state aligned with respect to the orbital angular momentum as an example and provide ion images of the signals that correspond to the population of the atomic magnetic sublevels ¦m¦ = 0, 1, 2. We show the contributions from the second-and fourth-rank state multipoles to the angular distributions of the atomic density matrix to be comparable in magnitude and significantly different in form.     

Reactions of excited I*(2P1/2) and unexcited I(2P3/2) iodine atoms in perfluoroalkyl radicals

A method of investigating reactions of excited and unexcited atoms is discussed. It is based on pulsed photolysis of molecules with simultaneous passage of laser radiation through the working medium. The method proposed is used to investigate the reactions that accompany the photolysis of the molecules RI(CF₃I, n-C₃F₇I, i-C₃F₇I). The rate constants of the recombination of iodine atoms into I₂ in the presence of RI molecules are calculated for the atoms I(²P_3/2) and I*(²P_1/2), as are the recombination constants of the radicals R into R₂ and with the atoms I*(²P_1/2) and I(²P_3/2) into the RI molecule. It is shown that the I(²P_3/2) atoms are much more active in the recombination into Ia and RI than the I*(²P_1/2) atoms. The role of the investigated reactions in the kinetics of a photodissociation iodine laser (PDIL) is discussed. The results are compared with the published data.     

Photolysis of NO2 at multiple wavelengths in the spectral region 200–205 nm

A study of the photodissociation dynamics of NO₂ in the 200–205 nm region using resonance enhanced multiphoton ionization (REMPI) in conjunction with the velocity map imaging technique is presented. We chose this region because it allowed the use of a single laser to photodissociate the NO₂ molecule and probe both the O(¹D₂) fragment using (2+1) REMPI via the 3p'¹P₁ state at 2 ×205.47 nm and the 3p'¹F₃ state at 2 ×203.5 nm, and the O(³P_J) fragments using (2+1) REMPI via the P_J states around 2 ×∼200 nm. Translational energy and angular distributions are extracted from the O(¹D) and O(³P) product images. A growth in the population of highly excited vibrational levels of the NO X(²Π) co-fragment is found as the dissociation wavelength decreases. These are compared with similar trends observed previously for other triatomic O-atom containing molecules. Detailed information on the electronic angular momentum alignment of the ¹D₂ state is obtained from analysis of the polarization sensitivity of the O(¹D) images using the two resonant intermediate states. The angular dependence of the potential energy in the exit channels is examined using long-range quadrupole-dipole and quadrupole-quadrupole interaction terms, from which molecular-frame multipole moments of the total angular momentum of the recoiling O atoms have been calculated. Comparison with the experimentally derived multipole moments is used to help provide insight into the dissociation mechanism.     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Interrelationship between the polarization moments of different parity upon optical pumping of atoms under magnetic resonance conditions: II. Theory

The mutual coupling between the polarization moments with ranks of different parity is theoretically considered. The manifestation of this mutual coupling has been revealed previously in experiments on magnetic resonance of optically oriented cesium atoms. The two well-known types of the coupling between the polarization moments are considered: the field coupling of these moments that occur due to the breaking of the hyperfine coupling between the electronic and nuclear moments of the alkali atom by the magnetic field and the light coupling of the moments due to the absorption of the pumping light by polarized atoms. The experimentally observed similarity in the shape of resonance signals of alignment and orientation upon circularly polarized pumping can be explained by the fact that, for alkali atoms, the generation of alignment by light at the wavelength of the D ₁ line is of low efficiency. Therefore, alignment arises mainly from orientation by means of either the field or the light coupling of polarization moments. For metastable 2³ S ₁ ⁴He atoms, no influence of the orientation on the alignment was observed because, in these atoms, the field coupling between the polarization moments is absent and the light coupling is not displayed because the generation of alignment by the circularly polarized pumping light is more efficient than the creation of alignment from orientation by means of light coupling of polarization moments.     

Possibility of separating the isotopes127I and129I with the aid of a photodissociation iodine laser

A method for separating iodine isotopes is proposed, based on the large difference between the rate constants of the excited I*(²P_1/2) and unaxclted iodine atoms with radicals CF₃ and with Cl₂ molecules, and on the possibility of selectively acting on the₁₂₇I atoms in the states²P_1/2 and²P_3/2 by radiation from a photodissociation R¹²⁷I iodine laser (λ = 1.315 μm). The possibility of separating the pure isotope¹²⁹I and the mixture¹²⁷I with¹²⁹I is investigated.     

Numerical Simulations of femtosecond-laser-induced dynamic alignment of molecules in the high-frequency off-resonance regime

The motion equation for ? between the molecular axis and laser polarization direction in a high-frequency off-resonance femtosecond laser field is deduced while simultaneously examining the effects of a permanent dipole moment and field-induced polarizability and hyperpolarizability to molecular rotation. Femtosecond-laser-induced dynamic alignment of CO, N₂, and Br₂ molecules are investigated by numerically solving the obtained rotation equation for the angle ?. The effects of the molecular permanent dipole moment and the field-induced polarizability and hyperpolarizability on the degree of alignment are presented at different intensities. Our computational results show that the dynamic alignment of molecules is primarily determined by field-induced polarizability and the second hyperpolarizability for the laser intensity range from 5 × 10¹⁴ to 5 × 10¹⁶ W/cm². The contributions of higher order correction terms to molecular alignment can usually be neglected. The polarizability-field interaction makes the angular distributions of a molecule have a maximum along the polarization axis and a minimum perpendicular to it. The role of the second hyperpolarizability keeps the molecular counts maximum along the laser polarization direction but minimum at an angle of 45° between the molecular axis and the polarization direction. There is also a second maximum of molecular counts perpendicular to the polarization axis. For CO, N₂, and Br₂ molecules, the dependences of laser-induced dynamic alignment on laser intensity exhibit completely different characteristics.     

Photofragment space distribution in the photodissociation of NaI in the spectral range 315–370 nm: the role of molecular axis rotation

The first determination of translational anisotropy parameters β in the photodissociation of NaI molecules in the spectral range 315–370 nm is reported. The anisotropy parameters were determined by the analysis of Doppler resolved absorption profiles of Na(²S_1/2) atoms produced in the photodissociation of NaI by linearly polarized light. The profiles were recorded for two orientations of the photolysis light: parallel and perpendicular to the direction of the probe beam. The value of the parameter β was obtained from a simultaneous fit of the profiles. The role of the rotation of the parent molecules on the branching ratio between parallel and perpendicular transitions in NaI during dissociation is discussed.     

Determination of the amplitudes and phases of the scattering matrix upon photodissociation of molecules

The photodissociation of RbI molecules via the second excited state under the action of circularly polarized radiation with a wavelength of 266 nm is studied. By using sub-Doppler Faraday spectroscopy, the angular distributions of the orientation of spins of Rb(5²S_1/2) atoms formed are studied and the anisotropy parameters describing these angular distributions are determined. Based on the anisotropy parameters, the ratio between the amplitudes of the inelastic scattering matrix elements corresponding to the two possible photodissociation channels is determined to be r_Ω=1/r_Ω=0=0.71±0.15, while the phase difference for these matrix elements is found to amount to Δφ = 173° ± 29°.     

Magnetic shielding and spin-rotation interaction in ground state alkali molecules

Precision measurements of nuclear magnetic dipole moments in alkali molecules are performed using atom-molecule exchange optical pumping. A comparison with measurements on alkali atoms gives the magnetic shielding differences between atoms and molecules and an approximate value for the spin-rotation interaction constant of the alkali molecules. It is reported on experiments on³⁹K₂ and⁸⁷Rb₂.     

Selective Stimulation of Chemical Reactions by Laser Excitation

Abstract

Among the many different applications to which lasers have been put, one of the more intriguing is their potential use in photochemistry - particularly, in inducing specific reaction pathways that are not accessible from conventional broad-band excitation. One of the first of such experiments was an attempt to use a ruby laser to carry out laser-flash photolysis of Br₂, analogous to Porter's work on I₂. Since the optical energy of the laser radiation (14,400 cm^?l) was less than the Br₂ dissociation energy, direct photodissociation was not possible; instead, excited Br₂? (³π_lu) molecules were produced, which were dissociated in subsequent collisions. The Br atoms formed in this way subsequently added to halogenated olefins such as C₄F₈ and C₄F⁷Cl.¹ The first actual laser flash-photolysis experiment, carried out in Porter's laboratory, used a Q-switched ruby laser to photodissociate dihydrophthalocyanine vapor at ca. 400°C; a multi-photon process is likely to be involved in this case.² As high-power infrared lasers became available, these were used to effect the decomposition of monia, ethyl chloride, and other molecules absorbing at 10.6 μm.^3,4 It seems clear, in retrospect, that all these reactions were essentially a laser-induced pyrolysis.     

Contribution of hexadecapole alignment to the polarization of quadrupole emission under laser excitation conditions

The linear polarization of quadrupole emission by the J=2-J ₀=0 transition under conditions of laser excitation in a gas medium is studied. Cases of excitation through dipole and quadrupole absorption of monochromatic laser radiation are considered. Taking into account the anisotropy of collisional relaxation, the contributions of polarization moments of the second and fourth ranks, i.e., those with usual and hexadecapole alignments, to the signal of linear polarization of quadrupole emission are calculated. The dependence of this signal on the laser frequency, the density of the gas medium, and the angles determining the orientation of the system of axes of observation of light polarization relative to a laser ray is studied. The numerical calculations of contributions of usual and hexadecapole alignment to the signal of linear polarization are made for the magnetic quadrupole transition J=2-J ₀=0 between the states 2p ⁵(² P ₁ ^2/0 )3p′[3/2]₂ and 2p ⁵(² P ₁ ^2/0 )3s 3s′[1/2]⁰ of neon atoms in the xenon atmosphere.     

Analysis of 205-nm photolytic production of?atomic hydrogen in?methane flames

We investigate the 205-nm photolytic production of atomic hydrogen in methane flames. This process represents a significant interference in two-photon, laser induced-fluorescence (TP-LIF) detection of atomic hydrogen in flames. Relative TP-LIF profiles of the photolytically produced H atoms were measured using a pump-probe technique in atmospheric-pressure, premixed CH₄/O₂/N₂ flames. A high-fluence, non-resonant, nanosecond pump laser created H atoms by photodissociating flame constituents, and a copropagating, non-perturbing picosecond laser probed the photolytically produced Hatoms via TP-LIF. Spatial profiles of photolytically produced H atoms indicate that both intermediate and product species contribute to the interference in all flames. Excellent agreement between simulated and measured interference signals is observed in the product region of the flames. Vibrationally excited H₂O is the dominant source of interference in the product region, but an additional contribution is attributed to vibrationally excited OH radicals. In the flame-front region, CH₃ is the dominant precursor, and photodissociation of C₂H₂ becomes increasingly important in rich flames. Mechanisms for sequential photodissociation of CH₃ and C₂H₂ are presented, indicating that complete dissociation at 205 nm of both precursors is feasible.     

Measurement of the electric quadrupole moments of 26–29Na

The nuclear electric quadrupole moments of the isotopes ²⁶Na, ²⁷Na, ²⁸Na and ²⁹Na were measured by -NMR spectroscopy in single crystals of LiNbO₃ and NaNO₃. High degrees of nuclear polarization were produced by optical pumping of the sodium atoms in a fast beam with a collinear laser beam. The polarized nuclei were implanted into the crystals and NMR signals were observed in the -decay asymmetries. Preparatory measurements also yielded improved values for the magnetic moments of ^27-31Na and confirmed the spin I=3/2 for ³¹Na. The results are discussed in comparison with large-basis shell model calculations. Received: 1 February 2000 / Accepted: 3 April 2000     

The photodissociation of UF6 using infrared lasers

The ir collisionless multiple photon absorption (cmpa) photodissociation of UF₆ is reported. Single frequency photodissociation is a accomplished with the focused output from a pulsed CF₄ laser operating at 615 cm^-1. When focusing the CF₄ laser output with a 7.5 cm f.1. lens, photodissociation is observed at laser energies as low as 5 mJ. By using a CO₂ laser (0.7 J, 1077 cm^-1) in concert with the CF₄ laser, significant enhancement (factors of 10–100) of the photodissociation yield is obtained at low CF₄ laser energies. Electronic emission is observed from the focal region, but only when both lasers are present.     

EUV and soft X-ray photoelectron spectroscopy of isolated atoms and molecules using single-order laser high-harmonics at 42 eV and 91 eV

Photoelectron spectroscopy of isolated atoms and molecules using single-order high-harmonics of Ti:Sapphire laser pulses (800 nm, 12 fs/30 fs) is demonstrated. Dielectric multilayer mirrors, SiC/Mg and Mo/Si, are used to isolate the 27th (42 eV) and 59th (91 eV) order harmonics, respectively. The obtained harmonics are characterized by valence and inner-shell photoelectron spectroscopy of Xe. The applications to two-color two-photon ionization of He and pump-probe spectroscopy of ultrafast photodissociation of Br₂, Br₂(C¹Π_u) → Br(²P_3/2) + Br(²P_3/2), are presented.     

纳米氦液滴-超低温化学研究装置

成功研制了新一代纳米氦液滴实验装置．氦液滴是由高压的高纯氦气通过超低温的脉冲阀绝热膨胀形成的,通过调节连接在超低温氦冷头上的脉冲阀的温度(10～30 K)及脉冲阀内氦气的背景压力(10～40 atm),氦液滴的大小在包含103到105个氦原子间连续可调,和传统的连续氦液滴束源比较起来,脉冲束源的强度提高了一个量级以上,提供了一个和商品化脉冲激光器结合使用,研究超低温条件下超流体氦中的化学反应动力学的机会．通过研究氦液滴中包裹的CH₃I分子的光解动力学来对仪器的性能进行测试,利用离子速度影像技术研究了超低温纳米氦液滴中包裹的CH₃I分子在252 nm下的光解动力学,光解产物甲基通过(2+1)共振增强多光子电离并运用离子影像进行检测,结果表明光解产物的平动能及角分布被氦原子环境有效的弛豫．也证实了运用脉冲氦液滴束源研究衰减光谱的可行性,通过对氦液滴中掺杂苯的衰减光谱的研究,还发现小于3%的衰减信号都可以被检测到,表明所研制的脉冲氦液滴束的稳定性及检测器的灵敏度都是很高的．