Two laser experiment

Figure 11-3. Effect of the pump laser at 250 nm with a time advance of 10 ns on the one laser REMPI spectrum of the probe laser [15]. Both spectra were recorded using the same intensity for the probe laser. The pump laser in the two laser experiment resulted in a maximum depletion of 20-25% in the region between 245 and 280 nm, and an enhancement of three decades at 220 nm. See text for a detailed explanation of the experimental method. (Reproduced with permission from J. Phys. Chem. 2004, 108, 943-949. Copyright 2004 American Chemical Society.)...

Figure 11 Transient absorption spectra obtained following one-laser photolysis (355 nm ) and two-laser photolysis (355 + 590 nm ) of all-rrans-retinol in air-saturated acetonitrile. In the one-laser experiment, the spectrum was obtained 1.9 (xs following the 355 nm pulse. In the two-laser experiment, the spectrum was obtained 1.9 ps after the 355-nm pulse and 400 ns following the 590-nm pulse. The kinetic behaviors observed at 570 and 610 nm are also shown. (From Ref. 18.)...

The MPI-MPD of butadiene provides an excellent example of case-1 multiphoton photodissociation [28]. This is a two laser experiment where one laser is used to prepare the ion in its ground state and the other is used to study its fragmentation. The sample gas is cooled by... 

A two photon experiment may therefore be carried out transitions between levels involving a state which can be photodissociated may be monitored by changes in the photodissociation yield. This can be achieved using either one laser (and hence one frequency) to supply both photons, or by using two lasers - one at low power to drive transitions with minimum power broadening, and one at high power to drive the photodissociation process. The two laser experiment is clearly capable of achieving better resolution. 

CO2 laser photon, see next section. A broad excitation spectrum (FWHM of 4 cm ) has been observed. In a two-laser experiment no hole burning eflFects have been obtained, i.e. the broad spectrum has a homogeneous appearance [1]. A straightforward estimate yields a lifetime of about 1 ps, from the homogeneous width of 4 cm Still, the spectrum is due to excitation without ensueing dissociation. Thus it makes no sense to talk of a predissociative lifetime. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

Figure 9.22 illustrates how a CARS experiment might be carried out. In order to vary (vj — V2) in Equation (9.18) one laser wavenumber, Vj, is fixed and V2 is varied. Here, Vj is frequency-doubled Nd YAG laser radiation at 532 nm, and the V2 radiation is that of a dye laser which is pumped by the same Nd YAG laser. The two laser beams are focused with a lens L into the sample cell C making a small angle 2a with each other. The collimated CARS radiation emerges at an angle 3 a to the optic axis, is spatially filtered from Vj and V2... 

Experiments on the sky. Two experiments have been carried out at the sky, using two laser installations built for the American and French programmes for Uranium isotope separation, respectively AVLIS at the Lawrence Livermore Nat l Lab (California) in 1996 and SILVA at CEA/Pierrelatte (Southern France) in 1999. The average power was high pa 2 x 175 W, with a pulse repetition rate of 12.9 and 4.3 kHz, a pulse width of 40 ns and a spectral width of 1 and 3 GHz. Polarization was linear. The return flux was < 5 10 photons/m /s (Foy et al., 2000). Thus incoherent two-photon resonant absorption works, with a behavior consistent with models. But we do need lower powers at observatories ... 

A series of measurements in which the pump wavelength is varied reveal that at some energies the oscillations predominate for times beyond lOps, whilst at others the decay of population by curve-crossing wins out within 400 fs or so. The time resolution of the experiment is in this example is determined by the convolution of the two laser pulse widths, here about 125fs. 

Consider, by contrast, a two-color experiment where the continuum is accessed by two laser fields with a well defined relative phase, a and , . A schematic illustration of the experiment envisioned is provided in Fig. 1 a, where we consider the specific case of excitation with one- and three-photon fields of... 

A scanner with two lasers for Cy3 and Cy5 labeling is fairly good enough for most of the microarray experiments. However, multiple lasers are necessary for simultaneous detection of all four nucleotide polymorphisms in chip-based SNPs detection. Besides, an extra third flurophore attached to a sequence that specifically binds to a linker region of the DNA spots could be used for spotting quality control. 

The furyl fulgide 172 has found use as a stable, recyclable actinometer for conventional photochemical experiments in the 313-366 mn wavelength range, where 173 = 0.20 and is independent of temperature and concentration245. It has also been developed as an actinometer in one- and two-laser flash photolysis experiments246. The colored form 173 can be converted back to 172 by simple exposure to visible light (equation 65). 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

A fascinating category of experiments can be found in Table IV. These are the use of lasers to determine thermodynamic parameters. These include calorimetry (43), enthalpies of vaporization and vaporization rates (44, 45), and heat capacities (46). Other laser experiments that can be found in Table IV include the use of CW laser spectroscopy to determine the iodine binding-energy curve (47), the study of vibrational line profiles to determine intermolecular interactions (48), two photon ionization spectrometry (49), a study of optical activity and optical rotatory dispersion (50) and the development of several experiments using blue diode lasers (57). 

Recently, there has been much interest in the development and application of multidimensional coherent nonlinear femtosecond techniques for the study of electronic and vibrational dynamics of molecules [1], In such experiments more than two laser pulses have been used [2-4] and the combination of laser pulses in the sample creates a nonlinear polarization, which in turn radiates an electric field. The multiple laser pulses create wave packets of molecular states and establish a definite phase relationship (or coherence) between the different states. The laser pulses can create, manipulate and probe this coherence, which is strongly dependent on the molecular structure, coupling mechanisms and the molecular environment, making the technique a potentially powerful method for studies of large molecules. 

In our experiment a sequence of two intense laser pulses is used to excite K atoms in an atomic beam from the 4.s- to the 4p state. Simultaneously, the pulses ionize the excited a-toms from the 4/7 state to the continuum via two-photon ionization (Fig. 1). Photoelectron spectra were taken at various delay times between the two laser pulses and at different laser intensities at a fixed delay time. 

R. de Vivie-Riedle and J. Manz Prof. Neumark s question about detecting the hole burning in the nuclear wavepacket of the electronic ground state is very stimulating. In this context, we have developed a scheme for detecting the hole in the wavepacket by a femtosecond chemistry laser experiment that involves two laser pulses Our explanation will be for the specific system K2, but more general applications for other systems are obvious ... 

Most SHG studies involve incident energies in the visible or near-infrared spectrum. Infrared SHG studies are hindered by the current lack of sufficiently sensitive IR detectors. However, the sum frequency generation (SFG) technique allows one to obtain surface-specific vibrational spectra. In SFG, two lasers are focused on the sample surface, one with a fixed frequency in the visible and one with a tunable range of IR frequencies. The sample surface experiences the sum of these frequencies. When the frequency of the infrared component corresponds to a molecular vibrational mode, there is an increase in the total SHG signal, which is detected at the visible frequency [66]. The application of such... 

In the experiment Na atoms are excited by two lasers from the ground state to the nd state in an environment cooled to 180 K at which point n = 86 for the 29d — 30p transition. The atoms are exposed to the thermal radiation for 20 / ,... 

A CARS experiment has recently been done to determine the amount of vibrational and rotational excitation that occurs in the O2 (a- -A) molecule when O3 is photodissociated (81,82). Valentini used two lasers, one at a fixed frequency (266 nm) and the other that is tunable at lower frequencies. The 266 nm laser light is used to dissociate O3, and the CARS spectrum of ( (a A), the photolysis product, is generated using both the fixed frequency and tunable lasers. The spectral resolution (0.8 cm l) is sufficient to resolve the rotational structure. Vibrational levels up to v" = 3 are seen. The even J states are more populated than the odd J states by some as yet unknown symmetry restrictions. Using a fixed frequency laser at 532 nm (83) to photolyze O3 and to obtain the products 0(3p) + 02(x3l g), a non-Boltzmann vibrational population up to v" = k (peaked at v" = 0) is observed from the CARS spectrum. The rotational population is also non-Boltzmann peaked at J=33, 35 33, 31 and 25 for v" = 0,1,2,3, and k, respectively. Most of the available energy, 65-67%, appears in translation 15-18% is in rotation and 17-18% is in vibration. A population inversion between v" = 2 and 3 is also observed. 

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

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