“Hole-burning spectra”. A new method for obtaining fine structure in absorption spectra of organic molecules

A new method is proposed for obtaining fine structure in broad-band absorption spectra of organic molecules in solid solutions at low temperature. This method is based on the laser burning of a series of narrow gaps in inhomogeneously broadened absorption spectra. A special spectral set-up has been developed for the measurement of the differential spectrum (a “hole-burning spectrum”) of “burnt” and “unburnt” samples. As an example the hole-burning spectrum of Zn-tetraphenylporphine in ethanol has been considered.     

Excited singlet state absorption spectrum with tunable dye lasers

A technique for measuring absorption from excited states with nanosecond lifetimes utilizing pulsed tunable dye laser probes is described. The method features a simple experimental set-up, and high signal-to-noise ratio. The excited singlet state absorption spectrum of one transition in acridine red is presented.     

CH3F absorption of the CO2-laser emission measured by the photoacoustic technique

Spectra of coincidence of CH₃F IR absorption with CO₂-laser emission at pressures of 2, 10 and 60 Torr were recorded by the use of a photoacoustic detection method in the whole range of CO₂-laser emission. The spectra show that CH₃F absorbs many CO₂-laser lines in the range 1084–1071 cm⁻¹ with the strongest absorption at 1046.80cm⁻¹, laser line P(20). Absorption is predominantly due to the fundamental of v₃, which is well spread over the whole laser emission range. The intensities of all absorptions rise with increasing pressure, but some absorptions change their relative intensity with respect to one another. In addition, the fine structure of the line spectrum, characteristic of lower pressure samples, disappears as pressure is increased.     

射流冷却的OCS分子高分辨VUV吸收光谱研究

The absorption spectrum of OCS molecules under jet-cooled conditions has been measured between 64150 and 65840 cm-1 with a highly resolved tunable vacuum ultraviolet (VUV) laser (about 0.2 cm-1), generated by two-photon resonant four wave mixing process. During the experiment, the VUV intensities prior and posterior to the OCS absorption were monitored simultaneously in order to increase the sensitivity of absorption measurement, and the absorption sensitivity △I/I was determined to be 1%. The measured spectrum is essentially similar to the S(1S) photofragment excitation spectrum under similar conditions, indicating that the dissociation to produce S(1S) fragment is the main pathway. The difference between these kinds of spectra, however, implies the existence of other dissociation channels in addition to the S(1S) pathway.     

Different molecular constituents in pheomelanin are responsible for emission, transient absorption and oxygen photoconsumption

Steady-state absorption and emission spectroscopies, oxygen activation and transient spectroscopy on a single sample of synthetic pheomelanin are compared. The absorption, emission and excitation spectra of pheomelanin depend on the molecular weight (MW) of the dissolved pigment constituents. While weakly-depending on MW, the maximum of the emission excitation spectrum is approximately 400 nm. The electron paramagnetic resonance oximetry measurements show a clear increase in oxygen uptake between 338 and 323 nm, and also reveal a local enhancement around approximately 370 nm. Pump-probe absorption spectroscopy reveals that photoexcitation of pheomelanin by UVA light generates a transient absorption peak in the visible and UV regions within the instrument response. The action spectrum for the formation of the 780 nm transient species peaks at approximately 360 nm. While both transient absorption bands show the same ultrafast decay component, the 780 nm peak completely vanishes on the 10s of picosecond time scale, but the UV band shows a kinetic evolution to a subsequent intermediate. While in a similar wavelength range, the maximum of the action spectrum derived from the transient data, the emission excitation spectrum and the action spectrum for photoconsumption all differ from one another, suggesting that the chromophore responsible for each is not that same. This raises concern about comparing the results from different photochemical methodologies for melanin, which is a specific case of comparing data on systems where molecular constituents are not well defined.     

Effective two-photon absorption cross section of heteroaromatic quadrupolar dyes: dependence on measurement technique and laser pulse characteristics

The linear and nonlinear optical properties of the heteroaromatic push-pull-push two-photon absorbing dye N-methyl-2,5-bis[1-(N-methylpyrid-4-yl)ethen-2-yl]-pyrrole ditriflate (PEPEP) are reported. The determination of the two-photon absorption (TPA) cross-section spectrum has been performed with different techniques: femtosecond TPA-white light continuum probe experiments, two-photon-induced fluorescence, and open aperture Z-scan measurements using both nanosecond and femtosecond laser pulses. The measured TPA cross sections and their wavelength dispersion show a marked dependence on the parameters of the laser pulses and on the measurement technique employed. These properties are discussed in terms of the different microscopic mechanisms that can contribute to the multiphoton absorption processes, with different weight depending on the measurement conditions and on the photophysical parameters of the dye.     

The detection of carbon monoxide by cavity enhanced absorption spectroscopy with a DFB diode laser

It has been proven that cavity enhanced absorption spectroscopy is a high sensitive spectral technique. The aim of our study was to apply this spectral technique to the detection of carbon monoxide with a narrow line width tunable DFB diode laser and high Q factor optical cavity. Absorption signals were extracted from a measurement recording the average of 20 highest light intensities that leak out of the cavity. The absorption spectrum of CO centered at 6354.18 cm⁻¹ was recorded; the experiment results indicate that cavity enhanced absorption spectroscopy could produce accurate high resolution spectrum. A detection sensitivity about 5.687 × 10⁻⁷ cm⁻¹ has been achieved in a 45 cm-long cell.     

Quantitative densitometric determination of reserpine and ajmaline in Rauwolfia Vomitoria by HPTLC

A direct densitometric method for the quantitative measurement of reserpine and ajmaline in whole roots and bark of roots of Rauwolfia Vomitoria, after extraction and separation by thin-layer chromatography, is described. The emission and excitation spectra of the reserpine and the absorption spectrum of the ajmaline were scanned. The linear range between peak area and concentration is given.     

Amplification of anharmonicities in multiphoton vibrational action spectra

The influence of one or several infrared laser pulses on the stability of bare and argon-tagged sodium chloride clusters is investigated theoretically by a combination of computational methods involving explicit molecular dynamics and properly calibrated unimolecular rate theories. The fragmentation spectra obtained by varying the laser frequency in the far-IR range is compared to the linear absorption spectrum resulting from the dipole moment autocorrelation function. Under appropriate laser field parameters, the action spectra are found to resemble the absorption spectra quite accurately in terms of positions, line widths, and even relative intensities. However, the action spectra exhibit residual and systematic redshifts of a few percent, which are partly due to the finite spectral bandwidth but are amplified by the progressive heating by the laser. A quantitative analysis suggests that these anharmonicity effects should generally arise upon multiple photon absorption.     

The spectrum of collision free triplets and its temporal evolution

A tandem laser experiment now enables one to follow the time history of the radiationless formation of triplets from inception to the final relaxed state known from flash photolysis. The TT absorption spectrum is measured shortly after excitation of the singlet and then at different times up to full relaxation at 14 μs. This spectrum is totally different from the well known flash photolysis spectrum, both in spread and its structure. The spectrum reveals both a relaxation process to thermalized triplets, as well as direct evidence for overall loss of triplets by a further radiationless process. Quantitative data can here be presented for these processes.     

Effect of low power laser irradiation on disconnecting the membrane-attached hemoglobin from erythrocyte membrane

In our previous study we found that low power laser irradiation improved the erythrocyte deformability, but the mechanism is unclear. The membrane-attached hemoglobin (Hbm) may be one of the determining factors for the erythrocyte deformability. We report here for the first time, that laser irradiation can reduce the Hbm contents in pig's erythrocytes, providing the explanation for the improvement of erythrocyte deformability. The decrease of the Hbm was proportional to the irradiation dose, but the relative change of Hbm was saturated around 35%. The 532 nm laser was more efficient at lowering Hbm than the 632.8 nm laser, consistent with the absorption spectrum of Hbm.     

Molecular two-photon spectroscopy with two synchronized tunable dye lasers

Two tunable dye lasers, with differing wavelength and polarization, were synchronized to produce a two-photon spectrum in gas phase benzene. Simultaneous absorption of two photons from one laser beam was precluded by choosing appropriate wavelengths. All possible ratios have been measured for a totally-symmetric two-photon transition demonstrating a strong analogy between two-photon absorption and Raman scattering.     

Application of the 2f-wavelength modulation technique for the measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy

The measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy was investigated using a low pressure atomizer and 2f-wavelength modulation (WM) detection. The measurements were supported by computer simulations. Compared with a direct absorption measurement, the relative absorption sensitivity for ⁶Li is considerably reduced when 2f-WM is performed in the center of the ⁶Li D1 fine structure component, but it will be enhanced, when the center of modulation is tuned to the maximum in the red wing of the 2f-WM line profile. The results of calculated 2f-WM line strengths were used to deconvolute overlapping lines and enabled the measurement of ⁷Li/⁶Li isotope ratios as large as approximately 2000. The Li content of a sample with a strong sodium chloride matrix was determined by isotope dilution.     

Fluorescence behavior of 7-hydroxycoumarine excited by one-photon and two-photon absorption by means of a tunable dye laser

The fluorescence spectrum of 7-hydroxycoumarine in ethanol excited by a pulsed tunable dye laser reveals different features when excitation proceeds via one-photon and two-photon absorption. In the former case the spectrum shows two peaks delayed in time by approximately 2 ns and characterized by different lifetimes. The relative intensity of these peaks is unaffected by significant changes in the dye concentration and therefore the formation of an “exciplex” species seems to be here confirmed. In the spectrum obtained by two-photon excitation the second peak at longer wavelength is absent. Results are presented for both room and liquid nitrogen temperatures as well as for other solvents such as glycerin and EPA. These results are considered to be important for the evaluation of absolute two-photon cross sections where the quantum efficiencies of one- and two-photon processes are assumed to be the same.     

Calibration of Polarization-Sensitive and Dual-Angle Laser Light Scattering Methods Using Standard Latex Particles

A polarization-sensitive laser light scattering (PSLLS) method and a dual-angle laser light scattering (DALLS) method have been studied for in situ measurement of submicrometer hydrosol and aerosol particles. By using standard monodisperse polystyrene latex particles suspended in water and air as test particles, calibration of systems built based on the above methods have been performed. The effects of light scattered by agglomerated aerosol particles (multiplets) were corrected by considering the fraction of multiplets as determined with an aerosol measurement technique using a differential mobility analyzer. The change in the measured intensities of scattered light with particle diameter was then determined by calculations based on Mie theory. It was shown that the PSLLS system can determine particle diameters as small as approximately 60 nm for the test hydrosol particles and approximately 100 nm for test aerosol particles, respectively. The DALLS system can determine smaller diameters than the PSLLS system for test particles with no light absorption. The change in scattered light intensities with particle diameter was also investigated by theoretical calculations with various refractive indexes and scattering angles. The PSLLS and DALLS systems promise to become routine measurement tools for absorbing and nonabsorbing particles, respectively. Copyright 2001 Academic Press.     

The vibrationless A<--X transition of the jet-cooled deuterated methyl peroxy radical CD3O2 by cavity ringdown spectroscopy

The nearly rotationally resolved spectrum of the A (2)A(')<--X (2)A(") 0(0)(0) transition of perdeutero methyl peroxy near 1.35 microm has been studied via pulsed cavity ringdown spectroscopy. Albeit, this is a weak transition, it is possible to observe the spectrum under jet-cooled conditions (approximately 15 K) by combining a source of narrow-bandwidth radiation (approximately 250 MHz) with a supersonic slit-jet expansion incorporating an electric discharge. The near infrared radiation was obtained by using stimulated Raman scattering and a pulsed, nearly Fourier-transform-limited Ti:sapphire amplifier seeded by a scanable cw Ti:sapphire ring laser. The experimental spectrum has been fitted using a model Hamiltonian that includes the rigid body rotation of an asymmetric top and the spin-rotation interaction. An excellent quality fit was obtained resulting in the determination of 15 molecular parameters characterizing the A and X states. Other results reported for CD(3)O(2) include an estimate of the radical concentration and the vibronic transition dipole from the observed absorption intensities. Details about the spectral linewidths are also discussed.     

Ca+-叔丁胺络合物的激光诱导反应

使用反射式飞行时间质谱仪, 研究了Ca⁺-叔丁胺络合物在激光诱导下的反应. 得到了反应的光解谱和作为波长函数的光解行为光谱以及各反应通道的分支比. 反应有两个通道, Ca⁺-与分子的解离通道和生成产物Ca⁺-NH₂的反应通道, Ca⁺-是主要产物, 而且在整个激光扫描的范围都存在, 并且在530～595 nm波段是唯一的产物. 反应的光解行为光谱显示出明显的无结构的峰, 分别对应于络合物的跃迁. 结合反应通道的分支比以及量化计算, 对这些峰进行了指认, 并初步探讨了反应的动力学机理.     

CH-stretching overtone spectroscopy of 1,1,1,2-tetrafluoroethane

We have recorded the vibrational absorption spectrum of 1,1,1,2-tetrafluoroethane (HFC-134a) in the fundamental and first five CH-stretching overtone regions with the use of Fourier transform infrared, dispersive long-path, intracavity laser photoacoustic, and cavity ringdown spectroscopies. We compare our measured total oscillator strengths in each region with intensities calculated using an anharmonic oscillator local mode model. We calculate intensities with 1D, 2D, and 3D Hamiltonians, including one or two CH stretches and two CH stretches with the HCH bending mode, respectively. The dipole moment function is calculated ab initio with self-consistent-field Hartree-Fock and density functional theories combined with double- and triple-zeta-quality basis sets. We find that the basis set choice affects the total intensity more than the choice of the Hamiltonian. We achieve agreement between the calculated and measured total intensities of approximately a factor of 2 or better for the fundamental and first five overtones.     

Absorption cross section and photolysis of OIO

Pulsed laser photolysis combined with transient absorption spectroscopy and resonance fluorescence was used to examine the photolysis of OIO at a number of wavelengths corresponding to absorption bands in its visible spectrum between approximately 530 and 570 nm. Photolysis at 532 nm was found to result in substantial depopulation of the absorbing ground state, enabling an estimate for the absorption cross section of OIO at 610.2 nm of (6 +/- 2) x 10(-18) cm2 molecule(-1) to be obtained. No evidence was found for I atom formation following photolysis of OIO at 532, 562.3, 567.9 and 573.8 nm, enabling an upper limit to the I atom quantum yield of < 0.05 (560-580 nm) and < 0.24 (532 nm) to be established.