Sub-Doppler optical one-photon spectroscopy of binary mixtures of neutral,mutually interacting gases with lasers: dispersion spectra

We consider in this paper the optical, sub-Doppler high resolution dispersion spectra for one-photon transitions of a mixture of two different interacting gases. The gases are supposed to be neutral and each molecule of the mixture interacts with molecules of the same kind and with molecules of different kinds, via hard collisions. Those interactions are restricted to binary collisions treated in the so-called impact approximation. We calculate the dispersion optical sub-Doppler spectra of a weak laser beam in the presence of a strong, saturating, counterpropagating laser one. The optical fields of both lasers are supposed to be travelling electromagnetic plane monochromatic waves. The angular frequencies ω₁ and ω₂ of those waves are taken, in general as different. An appropriate parametrizing procedure of all the physical quantities involved for treating this problem is taken.     

Adiabatic population transfer by acoustooptically modulated laser beams

We demonstrate coherent two-photon optical population transfer (STIRAP, J. Chem. Phys. 92, 5363 (1990)) between electronic states of atoms by use of two cw laser beams with high spectral resolution, which are modulated by an acoustooptical modulator (AOM) and provide a counterintuitive sequence of temporally delayed pulses of several hundred nanoseconds length. The efficiency of optical population transfer induced by this AOM-STIRAP method is shown in the lambda-type system of metastable Ne atoms, produced in a nozzle beam, where we compare directly spatially and phase separated pulses from cw laser beams (cw-STIRAP). As an application, efficient and selective transfer of population into a decaying electronically excited state of Na is demonstrated using the Doppler-free two-photon ladder-type transition 3S-3P-5S in a sodium cell. The experimental results are reproduced satisfactorily by use of a density matrix approach.     

Photofragment spectroscopy of molecular ions: design and performance of a new apparatus using coaxial beams

A new apparatus is described for the study of photofragment spectroscopy of molecular ions in a coaxial laser-ion beam configuration. Complementary to other developments in this field, the apparatus emphasizes the kinetic energy spectroscopy of the photofragments and is particularly designed to study direct photodissociation processes via repulsive potential curves. On the laser side, the experiment uses discrete lines in the visible and UV region (Ar⁺ laser, excimer laser). A detailed analysis of the experimental conditions is presented, in particular with respect to the attainable energy resolution in the photofragment spectra. The apparatus provides sufficiently intense ion beams (5–10 nA) with controlled energy resolution (ΔE = 100?500 meV) and angular collimation (ΔΘ = 5?10 mrad). The measured photofragment spectra of H ₂ ⁺ in the visible and UV region are in full accordance with the predictions of the design analysis.     

Non-linear effects by continuous wave cavity ringdown spectroscopy in jet-cooled NO2

A new method of high resolution cavity ringdown spectroscopy (CRDS) was recently developed in our laboratory, where a narrow line, continuous wave (CW) single-frequency laser is used instead of a pulsed laser. Here, we will first discuss the main differences between the `traditional' pulsed CRDS and CW-CRDS. Then, we will describe our results exploiting the high intracavity power that can be achieved with CW-CRDS. Using a single-mode Ti:Sa laser, we obtained CRDS spectra where the excitation power of a single cavity mode is close to 20 W. In the virtually collisionless regime of a supersonic slit jet, we observed saturation in some of the weak rovibronic transitions of NO₂ around 796 nm, as evidenced by loss of absorption intensity and formation of Doppler-free Lamb dips. In addition, in coincidence with absorption by these near infrared transitions, an appreciable fluorescence signal was detected in the visible range. According to our interpretation, this fluorescence is from NO₂ levels excited by two photons in a stepwise incoherent process, with a strongly allowed second step. Since the fluorescence spectrum has the same lineshapes as the CRDS absorption spectrum, it seems that the first transition step is the one limiting the overall two-step process. In addition, we also observed very narrow fluorescence features, not coincident with any absorption feature. These must be coherent (non-stepwise), Doppler-free, two-photon transitions. Interesting new questions arise from these preliminary data, and we believe that more measurements of this kind will provide new information about the rovibronic states of NO₂ in the dissociation region.     

Two-photon photosensitized production of singlet oxygen.

Singlet molecular oxygen (a(1)Delta(g)) has been produced and optically detected upon two-photon nonlinear excitation of a sensitizer with a focused laser beam. The experiments were performed using toluene solutions with either a substituted difuranonaphthalene or a substituted distyryl benzene as the sensitizer. The data indicate that the two-photon absorption cross sections of the difuranonaphthalenes are comparatively large and depend significantly on the functional groups attached to the chromophore. The time-resolved 1270 nm phosphorescence signals used to characterize the production of singlet oxygen are limited in much the same way as signals from other two-photon spectroscopic studies (e.g., weak signals that can be masked by scattered radiation). Nevertheless, the two-photon singlet oxygen signals also reflect the unique advantages of this nonlinear optical technique (e.g., depth penetration in the sample afforded by irradiation in a spectral region void of the more dominant one-photon linear transitions and spatial resolution afforded by irradiation with a focused laser beam).     

反式-4,4’-二（邻甲氧基苯乙烯基）联苯化合物的合成、结构及光物理性能

以Wittig-Homer反应合成了反式对称的4,4’-二(邻甲氧基苯乙烯基)联苯化合物, X射线衍射分析测定了化合物的晶体结构. 测定了其在不同溶剂中的线性吸收光谱、荧光发射光谱及荧光量子产率. 飞秒激光做光源研究了化合物的双光子光物理性能. 测试结果表明: 该化合物在蓝光波段有较强的荧光发射, 化合物具有较高的荧光量子产率和较大的双光子吸收截面, 具有双光子诱导蓝光发射光学特性.     

Vibrationally selective electronic energy transfer from metastable xe atoms to N2 and CO molecules

Formation of N₂(B) and CO(a', d) by collisions with Xe(³P_0,2) has been studied in the energy range 50–700 meV_c.m using crossed beams and beam/gas apparatus. Well-resolved product emission spectra were observed which showed high vibrational state specificity and low rotational excitation. These features are explained using semi-empirical interaction potentials.     

Reactive scattering of Sn from Cl2 and Br2 and a lower limit on D00 (SnBr)

Laboratory (lab) angular and time-of-flight (TOF) recoil velocity spectra of SnCl product formed by crossing a thermal Sn beam with a Cl₂ nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer—quadrupole mass filter detector unit. Product center of mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m. → lab transformation over measured beam speed distributions and the instrumental TOF resolution. The inferred distributions for the Sn + Cl₂ reaction are strikingly similar to those reported for the Li + Cl₂ reaction. In addition, SnBr product was observed from crossed beams of Sn and Br2 which establishes a lower limit for D⁰₀ (SnBr) of 131 kJ/mole.     

Effects of conjugation length and resonance enhancement on two-photon absorption in phenylene-vinylene oligomers

Two-photon excitation spectra have been recorded over the large spectral range of 540-1000 nm for five phenylene-vinylene oligomers that differ in the length of the conjugated pi system. The significant changes observed in the two-photon excitation spectra and absorption cross sections as a function of this systematic change in the chromophore are discussed in light of (1) the corresponding one-photon absorption spectra and (2) high-level density functional response theory calculations performed on analogues of these systems. The results obtained illustrate one way to exploit parameters that influence nonlinear optical properties in large organic molecules. Specifically, data are provided to indicate that when the frequency of the laser used in the two-photon experiment is nearly-resonant with an allowed one-photon transition, significant increases in the two-photon absorption cross section can be realized. This phenomenon of the so-called resonance enhancement allows for greater control in obtaining an optimal response when using existing two-photon chromophores, and provides a much-needed guide for the systematic development and efficient use of two-photon singlet oxygen sensitizers.     

Single sideband doppler-free double resonance stark spectroscopy using an acousto-optic modulator

An acousto-optic modulator driven at a single radio frequency Ω_RF, is used to modulate a cw single mode CO₂ laser oscillating at frequency Ω_L, producing a single sideband at Ω_L-Ω_RF. This system is used to demonstrate optical-optical double resonance using both co-propagating and counter propagating beams. Doppler-free Stark spectra of the molecules CH₃OH, CF₃I, H₂CS and CH₂NH are used to illustrate the technique.     

Optical differential mobility analyzer for micron size colloidal particles: theoretical approach

A loosely focused light beam is very useful for separating colloidal particles. For micron size colloidal particles suspended in a fluid and irradiated with a laser beam perpendicular to the direction of fluid flow, particles have a retention distance determined by their size and composition, the flow rate of fluid, and characteristics of the laser. An optical differential mobility analyzer for separating colloidal particles using a loosely focused beam is proposed theoretically and the concept of optical mobility is introduced. For the proposed optical differential mobility analyzer, particle trajectories and retention behavior are discussed and a transfer function described by operational parameters is derived. The possibility of separating particles of various sizes and different chemical compositions is discussed. It is proposed that the analyzer can be integrated into a microfluidic lab-on-a-chip system suitable for separating colloidal particles and biological cells at a very high resolution.     

Simultaneous Determination of Carbon Disulfide,Carbon Monoxide,and Dinitrogen Oxide by Differential Absorption Spectroscopy Using a Distributed Feedback Quantum Cascade Laser

Differential absorption spectroscopy with a distributed feedback quantum cascade laser source was used for the simultaneous determination of carbon disulfide, carbon monoxide, and dinitrogen oxide. The tunable laser, which provides higher resolution and sensitivity, makes the absorption spectroscopy more sensitive for CS₂. Furthermore, an important consequence of using a multiple-component spectral fitting with simulated reference signals is the reduction of noise and optical fringes. The results showed that CS₂, CO, and N₂O were simultaneously determined with accuracies of 2.6, 2.8, and 1.8%. The detection limit for CS₂ was estimated to be 10.5 parts-per-billion with a path length of 11.774?m, which indicates that differential absorption spectroscopy is sensitive for the multiple-component analysis of the atmosphere.     

Laser-assisted (e, 2e) collisions in helium

The influence of a strong laser field on the dynamics of fast (e, 2e) collisions in helium is analyzed in the asymmetric, coplanar geometry. The interaction of the laser field with the incident, scattered and ejected electrons is treated in a non-perturbative way, while the remaining interactions are treated by using first order perturbation theory. Detailed calculations are performed for an incident electron energyE _{k i}=600 eV, an ejected electron energyE _{k B}=5 eV and a scattering angle θ _A =4°. The influence of the laser parameters (photon energy, intensity and direction of polarization) on the angular distribution of the ejected electron is analyzed. We find that in general the triple differential cross sections are strongly dependent on the dressing of the projectile and the target by the laser field.     

Theory for optical assembling of anisotropic nanoparticles by tailored light fields under thermal fluctuations

In order to evaluate the assembling processes of arbitrary-shaped nanoparticles (NPs) by the irradiation of a tailored laser beam under thermal fluctuations, we have developed a “Light-induced-force Nano Metropolis Method (LNMM)” as a new theoretical method based on the stochastic algorithm in the energy region and the general formula of light-induced force. By using LNMM, we have investigated the change of configurations of silver NPs with anisotropic shapes under the irradiation of laser beams with various polarizations and intensity distributions (Gaussian beam and axially-symmetric vector beams) in an aqueous solution at room temperature. As a result, it has been clarified that silver NPs can be selectively arranged into a characteristic spatial configuration reflecting the properties of an irradiated laser beam (wavelength, intensity distribution, and polarization distribution), and that the assembled structures possess broadband spectra and exhibit a strong optical response to the irradiated laser beam through the optimization with the help of fluctuations.     

Crossed ablated beams: CAB

We have examined the feasibility of performing crossed molecular beam chemistry with pulsed beams ablated from surfaces. Crossed ablated beams (CAB) could be formed by laser photodissociation, photoejection or thermal desorption involving either adsorbate molecules or substrate. We note that: (i) reagents can be formed in high flux pulses with temporal widths as short as the laser duration, (ii) reaction product signal levels are several orders of magnitude higher than for conventional crossed molecular beam experiments, (iii) the short duration of the beam crossing allows the product time-of-flight spectra to display excellent translational energy resolution, (iv) the method lends itself to the generation of free radical beams, permitting radical-radical reactions to be studied under single collision conditions, (v) reactions with cross-sections as low as 0.01 Ų should be readily observable. Other attributes of CAB include aligned reagents or beams of complexes.     

Differential cross sections for fine-structure inelastic collisions of K(42P) with Ar,Kr and N2

We report measurements of differential cross sections for fine-structure inelastic collisions of potassium (4²P_3/2-4²P_1/2 with Ar, Kr and N₂. The experiment uses crossed molecular beams and a method to detect scattering angles by the analysis of Doppler shifts in laser induced fluorescence. The experimental results for KAr are compared with calculations.     

Multiphoton femtosecond phase-coherent two-dimensional electronic spectroscopy

In this paper the authors compare 400 nm one-photon and 800 nm two-photon two-dimensional Fourier transform electronic spectra of the organic laser dye Coumarin 102 in methanol using collinear optical pulse sequences and phase cycling. Results from the two different experiments show differences in the photon echo peak positions and shapes, reflecting differences in the two-photon and one-photon selection rules.     

Spectroscopy of the E band of SO2 with a frequency-doubled single-mode dye laser

The rotational structure of the E band of SO₂ between 32720 and 32900 cm^?1 has been investigated by absorption spectroscopy with reduced Doppler width in a cooled cell at 190 K using a frequency-doubled cw single-mode dye laser. More than 400 lines in this band could be assigned, starting from a former partial analysis by Hamada and Merer. Severe perturbations in the upper state impede the assignment and prevent the unambiguous identification of many other lines in the spectrum. From the measured lines term values and molecular constants of the upper levels are derived. A comparison with spectra of very high resolution, taken in a collimated supersonic SO₂ beam demonstrates that the actual line density is by far higher than the spectral resolution in the cooled cell.     

Synthesis and two-photon absorption of highly soluble three-branched fluorenylene-vinylene derivatives

Two new three-branched fluorenylene-vinylene derivatives were synthesized by triple Heck-type or Horner-Wadsworth-Emmons reactions. Their one-photon absorption and fluorescence as well as their two-photon absorption properties are reported. These compounds, which combine very high solubility in organic solvents, high fluorescence quantum yield and giant two-photon absorption cross-sections in the red-NIR region (up to 3660 GM, in the femtosecond regime) are promising candidates for both optical power limiting applications and two-photon laser scanning microscopy.     

Laser induced photodissociation of H+2 and D+2 ions

Photodissociation of H⁺₂ and D⁺₂ has been observed in a crossed beam experiment. A laser used as photon source. The ion and laser beam cross each other inside the laser cavity. The momentum spectra of the resulting H⁺ or D⁺ fragments are recorded with a mass spectrometer. From the spectra the excess kinetic energy is calculated. These values agree with the theoretically expected ones within the experimental error. From the measured intensity distribution the relative population for several vibrational states in the primary ion beam is calculated. Our values deviate from the usual assumed Franck-Condon pattern as well as from the values reported by Dunn. The angular dependence of the fragments is also measured. This dependence indicates a polarization of the primary beam perpendicular to its direction.