Laser Intensity Factor

In order to classify the various patterns as a function of the laser energy absorbed by the surface, the Laser Intensity Factor (LIF), is defined [11] ... 

The fluorescent components are denoted by I (intensity) followed by a capitalized subscript (D, A or s, for respectively Donors, Acceptors, or Donor/ Acceptor FRET pairs) to indicate the particular population of molecules responsible for emission of/and a lower-case superscript (d or, s) that indicates the detection channel (or filter cube). For example, / denotes the intensity of the donors as detected in the donor channel and reads as Intensity of donors in the donor channel, etc. Similarly, properties of molecules (number of molecules, N quantum yield, Q) are specified with capitalized subscript and properties of channels (laser intensity, gain, g) are specified with lowercase superscript. Factors that depend on both molecular species and on detection channel (excitation efficiency, s fraction of the emission spectrum detected in a channel, F) are indexed with both. Note that for all factorized symbols it is assumed that we work in the linear (excitation-fluorescence) regime with negligible donor or acceptor saturation or triplet states. In case such conditions are not met, the FRET estimation will not be correct. See Chap. 12 (FRET calculator) for more details. 

For the measurement of small absorption coefficients or refractive indices, it is often advantageous to place the probe inside the laser cavity ). The sensitivity is then increased by a factor which depends on the quality of the Q factor of the cavity and which can be very large (about 100 or more), since quite small changes in total absorption may cause large changes in laser intensity, especially if the laser is operated close above threshold. 

The high laser intensity enables molecular transitions to be measured even when their Franck-Condon factors are small, so that the fluorescence progression can be followed up to high vibrational levels, thus considerably increasing the accuracy of the molecular constant determination. It furthermore permits fluorescence measurements at low pressures. 

If there is no fluctuation of laser intensity, we have to measure /q only once. Actually, the envelope of laser pulses changes in a relatively long time range (typically from several minutes to a few tens of minutes) because of the change of environmental factors such as room temperature and coolant temperature. There is also an intensity jitter caused by factors such as the mechanical vibration of mirrors and the timing jitter of electronics. Furthermore, in our system, the laser system is located about 15 m from the beam port to prevent radiation damage to the laser system. (Later, it was moved into a clean room, which was installed in the control room to keep the room temperature constant and to keep the laser system clean. The distance is about 10 m.) Therefore it is predicted that a slight tilt of a mirror placed upstream will cause a displacement of the laser pulse at the downstream position where the photodetector is placed. 

If we use an ns probe pulse, we can tune its wavelength resonant to one particular vibronic transition. In this case, the LIF signal reflects the population of a single vibrational level involved in the WP. By scanning the wavelength of the probe pulse, we can observe the population distribution of the eigenstates involved in the WP. The peak intensities of the LIF signal are influenced by the Franck-Condon factors and the probe laser intensities, so that the relevant corrections are necessary to obtain the population distribution. 

While the amplitudes are hard to compare at different wavelengths (and the amplitudes in Fig. 3a are therefore only rough), these quantities are better defined at a given wavelength. We found that for long Apt their relative values (amplitude/average signal) did not depend on the probe laser intensity, which was varied over a factor of 3. 

G. Gerber The observed molecular ATI (above-threshold ionization) in Na2 occurs for laser intensities above 10l2W/cm2. The observation that no additional fragmentation channels (except the 2EJ channel) open up in Na2+ might explain why in femtosecond cluster experiments additional ion fragmentation channels do not show up. Within the femtosecond interaction time even under ATI conditions the lowest Franck-Condon factor (FCF-allowed) vibrational levels are the only ones observed. At even higher intensities when multiple ionization of clusters occurs, the situation can be different. This has not yet been investigated in detail. 

The laser output intensity of the C153 and R6G ORMOSIL gels was studied as a function of the number of laser pump pulses. Both materials could be pulsed for more than 3000 shots with a reduction of the emission amplitude of about a factor of four. Specifically, the C153 gel laser intensity decreased by a factor of 6 after more than 6000 pulses of 500 MW/cmA The plot of the intensity versus number of shots has a double exponential decay. This phenomenon is not yet completely understood, but it could be associated with microscopic phase separation in the medium. The R6G decay plot shows that the intensity undergoes a 90% reduction after 5300 laser pulses. 

The peak of the intensity of the circularly polarized light is half of that of the linearly polarized light. But this is not sufficient for the effective laser intensity in the ionization experiments. A factor of 0.65 has been estimated on the basis of the ADK calculation of Xe ionization rates [27]. The factor, 0.75 was experimentally obtained here by comparing the Isat of Xe for linearly polarized light to that for circularly polarized light [12]. 

A Mathematica calculation of Franck-Condon factors that determine electronic transition intensities of I2 is presented in Chapter III, and program statements for this are illustrated for I2 in Fig. III-6. In this fignre, note the dramatic differences between the intensity patterns predicted for the harmonic oscillator and Morse cases and compare these patterns with those seen in your absorption spectra. If yon have access to this software, yon might examine the changes in the harmonic-oscillator and Morse-oscillator wavefnnctions for different v, v" choices. A calcnlation of the relative emission intensities from the v = 25, 40, or 43 level conld also be done for comparison with emission spectra obtained with a mercury lamp or with a krypton- or argon-ion laser, hi contrast to the smooth variation in the intensity factors seen in the absorption spectra, wide variations are observed in relative emission to v" odd and even valnes, and this can be contrasted with the calcnlated intensities. Note that, if accnrate relative comparisons are to be made with experimental intensities, the theoretical intensity factor from the Mathematica program for each transition of wavennmber valne v shonld be mnltiphed by v for absorption and for emission. ... 

Here, Qa and Qb account for the different optical properties of the fluo-rophores that distinguish A and B as well as the laser intensity and other instrumental factors. K = kab/kba, and td = w jAD is the characteristic diffusion time for a Gaussian excitation intensity profile with exp(—2) radius w, and S is the area of the laser spot. It is readily shown that for equilibrium systems Gab (t) = Gba (t) due to the fact that kabCB = kbaC [25]. The NESS fluxes can be obtained from the initial slope of the correlation functions. 

Figure 3. The static structure factor, S(k), for various laser intensities. The peak in S(k) corresponds to the roton minimum in the energy spectrum. The para-maters are the same as those used in Fig. 2.

Extension of our technique described here is of course limited by various factors. The laser intensity cannot be increased greatly because of the damage of lenses and mirrors of the microscope. Therefore, the concentration of a fluorophore to be bonded as a probe must be rather high, especially when its fluorescence quantum yield is low. Thus, it is difficult to say definitely the smallest size of a particle which is amenable to the decay measurement, but in a fortunate case, a particle as small as 1 um can be examined (8). When a particle is larger than a few tens ym, it is also possible to examine spatially heterogeneous decays within the particle (see Fig.2). 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

The rate of conventional (single-center) two-photon absorption depends on the square of the focussed laser intensity, and as long ago as 1968 Gontier and Trahin showed that in the absence of accidental resonances an intensity factor of (/// ) is introduced for each additional photon involved in a multiphoton atomic excitation process. The constant / is a characteristic irradiance whose value depends on the sample, and corresponds to the situation where perturbation theory breaks down and all multiphoton processes become equally feasible. A similar trea ment of molecules leads to an intensity factor per photon of y = where If is an irradiance that... 

Solutions without Pc gave very weak luminescence at 7(X) nm, detected only when high fullerene concentrations and laser intensities were used. Addition of as little as 10 M Pc increased the intensity by an order of magnitude. Increasing the Pc concentration caused a further increase in intensity. The lifetime of the 700 nm luminescence was shorter than that at 1270 nm by a factor of 2 Fig. 1 shows the relationship in similar lifetime relationships occur in solvents with a wide range of singlet oxygen lifetimes and in the presence and absence of quenchers (Fig. 2). In all cases, the intensity at 700 nm was... 

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