Infrared laser-excitation

Direct spectroscopic observation of the postulated diradical intermediates has not been possible so far. Thus, multiphoton infrared laser excitation of tetramethyldioxetane in the gas phase failed to detect diradical intermediates with lifetimes greater than about 5 nsec.Picosecond spectroscopy limited the lifetime of a diradical intermediate, if formed, to less than about lOpsec in the 264-nm pulsed photolysis of tetramethyldioxetane in acetonitrile, using a mode-locked neodymium... 

Bimolecular Reactions of Infrared Laser Excited Molecules... 

Tunable, narrow bandwidth lasers provide a very convenient method for obtaining selective vibrational excitation. A number of reactions of infrared laser-excited species have been studied to date, and are Usted in Table I. Also included is the frequency of the pump laser, the ground-state heat of reaction, and the effect of vibrational excitation on reaction rate. Reactions in Table I are listed according to increasing molecular complexity. 

Fig. 8. Schematic drawing tandem mass spectrometer in Z configuration for coaxial infrared laser excitation. Aperture lenses have been omitted for clarity. Comptonents (a) Nozzle, (b) skimmer, (c) electron gun, (d) and (i) quadrupole mass filter, (e) and (g) quadrupole deflector, (f) octopole ion guide, (h) CO2 laser, (j) ion conversion dynode, and (k) secondary eiectron multiplier. ...

To construct appropriate initial conditions that mimic the infrared laser excitation of a specific normal mode into its first excited state, we consider a molecule containing N atoms with Cartesian coordinates r = ( j... 3 ) and vibrational velocities V= (4i---43n). The total vibrational energy of the molecule is... 

In the lower part of Fig.3, this is explained in the potential diagram. A few rotational states are shown both for the vibrational ground state (0,0,0) and the vibrationally excited state (0,0,1) (asymmetric stretch mode). The infrared laser excites a particular rotational transition from the vibrational ground state (000) to the asymmetric stretch mode (001) in the wavelength range around 2.7ym, in the example from rotational state 3o3 to rotational state 4q4. The rotational quantum numbers are J,Kg and for total J and its projection on the A- and C-axis. Other rotational states can be prepared, simply by scanning the infrared laser to other rotational transitions. 

H2O, the spectrum can be identified and a few transitions are indicated. The lower trace shows the enhanced formation of OH as a function of the infrared laser wavelength, with the OH probe laser tuned to the Ri(0) transition. The comparison of the upper and the lower trace reveals that there is enhanced OH formation whenever the infrared laser excites a particular rotational transition from (0,0,0) to (0,0,1). The signals in the lower trace are due to the photodissociation of H2O from single rotational states ... 

The study of infrared laser excitation in D2 at a ruthenium(OOOl) surface shed light on the importance of timescales for controlling efficiency... 

Continuous wave (CW) lasers such as Ar and He-Ne are employed in conmionplace Raman spectrometers. However laser sources for Raman spectroscopy now extend from the edge of the vacuum UV to the near infrared. Lasers serve as an energetic source which at the same hme can be highly monochromatic, thus effectively supplying the single excitation frequency, v. The beams have a small diameter which may be... 

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

Quack M 1989 Infrared laser chemistry and the dynamics of molecular multiphoton excitation Infrared Rhys. 29 441-66... 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

The data obtained in tlie infrared-diode-laser-probe studies described above provides quenching infonnation at a given substrate donor energy E. By varying tlie laser excitation wavelengtli for production of vibrationally hot... 

Flynn G W, Michaels C A, Tapalian C, Lin Z, Sevy E and Muyskens M A 1997 Infrared laser snapshots vibrational, rotational and translational energy probes of high energy collision dynamics Highly Excited Molecules Relaxation, Reaction, and Structure ed A Mullin and G Schatz (Washington, DC ACS)... 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

The purpose of this article is to review some of the current endeavors in this developing field. To maintain brevity, the focus is on recent studies carried out in our own laboratory and in conjunction with Professor M.T. Bowers at the University of California at Santa Barbara, with emphasis on the use of kinetic energy release distributions and infrared laser multiphoton excitation to probe potential energy surfaces for the reactions of atomic metal ions with alkenes and alkanes. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

J. Hicks, D. Andrews-Wilberforce, and G. Patonay, A near-infrared laser diode excitation source for an SLM 8000CI luorometer, Anal. Instrum. 19,29-47 (1990). 

Many molecules which have absorption bands in the wavelength region of existing laser lines can be excited by absorption of laser photons into single isolated rotational-vibrational levels of the electronic ground state 1W>-103) (jn the case of infrared laser lines) or of an excited electronic state (with visible or ultraviolet lines)... 

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