Lasers molecular transitions

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. 

The obtainable small laser linewidth with tunable wavelength (Section II) improves selection of different molecular transitions and in many cases the selective population of a single excited rotational level can be achieved. 

One of the main goals of the crossed-beam experiment is to measure the internal energy AEvlh rol transferred to the molecule. In principle, this is possible in either of two ways. First, the scattered molecules could be detected and their product-state population analyzed. Infrared emission or absorption techniques may be considered, similar to those used in cell experiments.13 21 Although such studies would lead to the most detailed results (at least for polar molecules), under crossed-beam conditions they are impossible for intensity reasons, even if the possibility of measuring differential cross sections is renounced and the molecules in the scattering volume itself are detected. Detection via electronic molecular transitions may be invisaged. Unfortunately, the availability of tunable lasers limits this possibility to some exotic molecules such as alkali dimers. The future development of UV lasers could improve the situation. Hyper-Raman... 

Since the laser magnetic resonance experiment relies on a chance near-coincidence between a laser line and a molecular transition frequency, and the range over which spectroscopic transitions can be magnetically tuned is often quite small, it is desirable to have a large number of FIR laser lines available. This is now seldom a major problem, and table 9.1 lists a restricted sample of FIR laser lines that have been used for magnetic resonance studies. It is, of course, necessary to be able to measure the FIR frequency accurately and this is accomplished in Evenson s laboratory by measuring the beat... 

On the contrary, if wc have lasers polarized in perpendicular directions to each other, we point the z-axis in the direction of polarization of second laser and consider molecular transitions in a. sequence J" = 7, I = 8 —> Jo = 8, then we... 

Figure 6. The probability distribution of molecular axes for two step laser excitation of diatomic- molecules. For case a we have angular momentum quantum number sequenc e 7-8-9 in molecular transitions, but for case b we have 7-8-8 sequence. Mutual laser polarization for both cases is shown in the figure.

Quantum control methods make use of the time- and frequency dependence of the external laser field, usually assuming that the spatial dependence of the coupling between two electronic energy surfaces of a molecule is constant. In this work we ask what may be the influence of the spatial dependence of the coupling and can it be also used for steering molecular transitions ... 

The effects of a laser-induced molecular transition is most easily discussed within the perturbation approximation. Since the dipole couplings between molecular levels is often weak enough to allow the transfer of only a minute fraction of the ground state population into excited levels, working within the perturbative frame is often justified. 

In the second part of this work, we addressed the problem of how to use the above described effects of a space-dependent interaction to steer molecular transition. We proposed a model that allows us to induce a space-dependent coupling between two molecular potentials via a steady-state coupling to a third potential surface. By changing the frequency and intensity of the steady state laser, we can shape the space-dependence of the coupling. We illustrated the method with an example of three coupled harmonic oscillators and showed how displacement and width of the excited wave packet can be controlled. 

Studies of the photodissociation dynamics of chlorinated benzene derivatives have been reviewed. Photodissociation of chlorobenzene at 266 nm has been investigated by the crossed laser-molecular beam technique, and a hot molecule mechanism is considered probable. Similar studies have been carried out for bromobenzene and p-bromotoluene, which show that for each of these molecules the dissociation is fast and the transition dipole moment is almost perpendicular to the C-Br bond. In deoxygenated aqueous solutions, 254 nm photolysis of chlorobenzene yields phenol and chloride ions as the main products, along with benzene, phenylphenols and biphenyl.lodo-benzene adsorbed on sapphire(OOOl) at 110 K undergoes C-I bond cleavage when irradiated at 193 nm. ... 

In these equations the position of the molecule is described by the vector R the wavevectors of the two beams of modes r2 and are k2 and k3 respectively, with ( 2) and (q3) the corresponding mean photon numbers (mode occupancies) and is a unit vector describing the polarization state of mode rn. In deriving Eqs. (120) and (121), the state vectors describing the radiation fields have been assumed to be coherent laser states, and so, for example, (<72) = (oc n a(2 ), where a ) is the coherent state representing mode 2 and h is the number operator a similar expression may be written for (<73). Also, the molecular parameters apparent in Eqs. (120) and (121) are the components of the transition dipole, p °, and the index-symmetric second-order molecular transition tensor,... 

In the cooperative case, the two molecular transitions are separately allowed under well-known two-photon selection rules, since each molecule absorbs one laser photon and either emits or aosorbs a virtual photon. In the same way, the distributive case provides for excitation through three-and one-photon allowed transitions, and may thus lead to excitation of states that are formally two-photon forbidden. (In general, it is sufficient to stipulate that both transitions involved in the distributive mechanism are one-photon allowed since, with the rare exception of icosahedrally symmetric molecules, all transitions which are one-photon allowed are of necessity also three-photon allowed (Andrews and Wilkes 1985).)... 

For the single-beam cases, where the laser photon frequency is at the mean of two molecular transition frequencies, resonance enhancement occurs if either species involved in the process has an energy level matching one of those indicated in Fig. 10 by the broken lines. In the double-beam cases, the requirement for the mean of the two laser frequencies to match the molecular transition frequency provides additional freedom for one of the lasers to be tuned to one of the resonance levels shown in Fig. 11. Below we examine each case in more detail. 

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

There are two typical spectroscopic experiments. In the first, both lasers are in resonance with atomic (laser 1) and molecular (laser 2) transitions and the monochromator (Fig. 2) is scanned to record the laser-induced fluorescence. In the second type of experiment, the monochromator is used as a filter and is not scanned while the second laser wavelength is changed. This second type of experiment is called a laser excitation scan since a fluorescent signal is detected by the PMT only when laser 2 is in resonance with a molecular transition. In this case, the scanning laser can be broadband for survey work or single mode for high-resolution experiments. 

To illustrate the method described above, let us consider now one particular example (see Fig. 6). The diatomic molecule, for example Na2, is excited in two steps by two weak linear polarized lasers, which arc at an exact resonance (A = 0) — first one with the hrst molecular transition, but the second one with the second molecular transition. In the first case lasers are polarized parallel to each other. 

On the contrary, if we have lasers polarized in perpendicular directions to each other, we point the 2 -axis in the direction of polarization of second laser and consider molecular transitions in a sequence J" = 7 —t J = 8 —> J 2 = 8, then we have molecular axes distribution in the second excited state as shown in Fig. Qh. Now molecular axes are very strongly aligned in the plane that is perpendicular to the axis of quantization. This example demonstrates, how efficiently we can manipulate the molecular axes distribution just by varying angular momentum of the states excited by laser beams and mutual polarization of lasers. 

A popular technique is that of light (or photon) assisted OMVPE. Typically, a mercury lamp may be used to provide light at 185 and 254 nm, or a laser tuned to a specific wavelength may be used. In the case of UV assist, either the UV is used to assist with reactant decomposition — at or above the deposition plane, or the light is focused onto the substrate to promote reactivity and surface mobility. A laser, selectively tuned to a specific molecular transition, may be employed to promote molecules to an excited... 

The optothermal spectrum will faithfully represent the absorption spectrum provided that the molecules do not fluoresce prior to arrival at the detector. Fluorescence is not a problem because infrared fluorescence lifetimes are of the order of milliseconds. The transit time from the laser-molecular beam interaction region to the detector is tens of ps. Furthermore, it is possible to determine if the absorbing species is a stable monomer, or weakly bound cluster. When a stable monomer absorbs a photon, the amount of energy measured by the bolometer increases. When a weakly-bound species absorbs a photon greater than the dissociation energy, vibrational predissociation will take place and the dissociating fragments will not hit the bolometer element. 

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