Detection laser spectroscopy

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Mohebati A., King TA., Remote detection of gases by diode laser spectroscopy, J. Modem Optics 1988 35 (3) 319-324. 

Photolysis of DMDAF in benzene containing methyl alcohol gives the ether expected from the reaction of the singlet carbene. Monitoring this reaction by laser spectroscopy reveals that the detected transient reacts with the alcohol with a bimolecular rate constant very near the diffusion limits. In contrast, the transient reacts with triethylamine at least 100 times more slowly than it does with alcohol (Table 7). This behavior is inconsistent with identification of the transient as the cation or radical and points to its assignment as the singlet carbene. 

D. O. Landon and R. D. Compton, Laser spectroscopy and parallel detection technology, Tracor... 

Levenson, M. D., and Eesley, G. L. 1979. Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy. Appl. Phys. 19 1-17. Librizzi, R, Viapianni, C., Abbruzzetti, S., and Cordone, L. 2002. Residual water modulates the dynamics of the protein and of the external matrix in trehalose-coated MbCO An infrared and flash-photolysis study. J. Chem. Phys. 116 1193-1200. 

The pump and probe pulses employed may be subjected to a variety of nonlinear optical mixing processes they may be prepared and characterized by intensity, duration, spectral band width, and polarization. They may arrive in the reaction chamber at a desired time difference, or none. The probe pulse may lead to ionizations followed by detections of ions by mass spectrometry, but many alternatives for probing and detection have been used, such as laser-induced fluorescence, photoelectron spectroscopic detection, absorption spectroscopy, and the like. 

Laser state-to-state techniques include both the application of highly sensitive laser spectroscopy for internal state-resolved detection of molecules in the gas phase, e.g., desorbing or scattering from a surface, and second, for laser pumping an initial state prior to interaction with a surface. To date, laser detection of internal states has been widely applied in gas-surface dynamics experiments, while those involving optical state preparation techniques have only been applied in a limited fashion. 

Silylene SiH2 was directly detected during the pyrolytic decomposition of silane (and ethylsilane EtSiII3) for the first time by intracavity laser spectroscopy in 198865. The high detection sensitivity of this experiment allowed the pyrolysis mechanism to be examined under conditions that optimized film growth rather than spectroscopic conditions. 

C. Yamada and E. Hirota, Phys. Rev. Lett., 56, 923 (1986). Detection of the Silyl Radical SiH3 by Infrared Diode-Laser Spectroscopy. 

Silicon dicarbide has been identified by Thaddeus et al. (1984) as a circumstellar molecule on the basis of 9 hitherto unassigned millimeter wave lines observed in the late type star IRC + 10216. The molecule is the first ring molecule detected in space, and its rotational spectrum is that of a near prolate asymmetric top with C2v symmetry. The molecule had been detected in the laboratory prior to the interstellar detection by optical laser spectroscopy (Michalopoulous et al. 1984). 

The schematic view of the Mainz apparatus for collinear laser spectroscopy, installed at Isolde is given in fig 4. The 60 keV ion beam is set collinear with the laser beam, then accelerated (or decelerated) and finally neutralized in charge exchange cell. By Doppler tuning the atomic absorption is set resonnant with the stabilized laser frequency, and the fluorescence emitted is detected. 

Intracavity dye laser spectroscopy (IDLS) can be a powerful technique for detecting trace species important in combustion. The technique is based on the phenomenal sensitivity of a laser to small optical losses within the laser cavity. Since molecular absorptions represent wavelength-dependent optical losses, the technique allows detection of minute quantities of free radicals by placing them inside the laser cavity and monitoring their effect on the spectral output of the laser. 

Infra-red chemiluminescence has been used to measure the vibrational state distributions of CO2 formed by reaction on Pt and Pd surfaces [45-50]. While the detection sensitivity of infrared chemiluminescence does not approach that of LIF or REMPI, it is an attractive way to probe molecules such as CO2 where there is substantial vibrational excitation and REMPI schemes are not available. In some cases Doppler measurement of the translational energy release can be achieved, giving direct information on the translational energy release of vibrationally excited C02 [45]. Recently infrared diode laser spectroscopy has been used to detect vibrationally excited CO2 from CO oxidation over... 

Zucker, R.M. and Jeffay SC. Confocal Laser Scanning Microscopy of Whole Mouse Ovaries Excellent Morphology with Apoptosis Detection and Spectroscopy. Cytometry 2006 69A August 2006 930-939 (COVER). 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

When photo-electron transfer is used with a dye, such as eosin or dibromo-fluorescein in combination with NADH (and the dithionite concentration is low, ca 4 X 10" m), no dissociation of proteins is detected. Kinetic laser spectroscopy helped Syrtsova et al. [9] to follow electron transfer from the Fe protein to the MoFe protein and it was shown that, unlike the situation with dithionite, Fe protein in the complex with MoFe protein could undergo reduction by the photodonor as efficiently as in the free state in solution and electron transfer proceeds in the complex of two proteins without dissociation. 

A corresponding F2/CH3OH/O2 system has been used in discharge flow studies or alternatively the F2/H2O2 system. Detection methods include absorption [88], and i.r. diode laser spectroscopy [89] for time resolved studies and laser magnetic resonance for flow systems [90]. 

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