Single-mode excitation

We wish to add that there exists a wide variety of literature that considers the opposite case of monochromatic excitation by an infinitely narrow line causing velocity selection, such as [261, 268, 269, 320, 362] and the sources quoted therein. This description has been developed basically in connection with laser theory it refers most often to stabilized single-mode excitation. The intermediate case between monochromatic and broad line excitation is the most complex one, requiring integration over the modal structure of the laser inside the bounds of the absorption contour [28, 231, 243]. 

As mentioned above, rotations play a major role in CET. Most of the CET occurs via a T/R mechanism in which the translational energy goes into rotation. This is confirmed in a detailed study of single mode excitation [14] where each mode of the benzene was pumped separately. It was found that the values of a]i are a factor of 3-5 larger than the values of d- The conclusion that rotations are the major contributors to the values of an in all specific-modes excitation energies is supported by the work of Rosenblum etal. [15] who found that in SO2 rare gas collisions, rotations are the major energy transferring mode. 

Single mode generation can be achieved by carefully controlling the frequency and wavenumber bandwidths of the excitation. The frequency bandwidth can readily be limited by employing windowed toneburst excitation signals [2] while the wavenumber bandwidth is... 

Figure 10-14. Inset Phololumincsccncc spectrum for low excitation pulse energy EP Main part (a) displays the spectrum for pump pulse energies well below the lasing threshold while (b) shows the spectrum obtained lor excitation with a pump energy close to the lasing threshold (c) presents the single mode-lasing spectrum emitted when the device is pumped well above threshold. The dashed lines indicate the zero line which is arbitrarily shifted in case of (b) and (c).

Figure 10-15. Output vs. input energy characteristic of our laser device. The horizontal dashed curve indicates the zero line. A clear laser threshold behavior at an excitation pulse energy ol 1.5 nJ is observed. Below the lasing threshold only isotropic phololuminesccncc is entitled. Above threshold the device emits low divergence single mode laser emission perpendicular to the surface, as schematically shown in the inset. The laser light is polarized parallel to the grating lines.

The experimental set-up for the FCS measurement is illustrated schematically in Figure 8.6. A CW Ar laser (LGK7872M, LASOS lasertechnik GmbH) at 488 nm was coupled to a single mode optical fiber to isolate the laser device from an experimental table on which the confocal microscope system was constructed. This excitation laser light transmitted through the optical fiber was collimated with a pair of lenses, and then was guided into a microscope objective (lOOX, NA 1.35, Olympus). 

As evidenced by the correlation of the BH model with the experimental data, Figure 2.4, the model is only in qualitative accord with the experiment. Clearly, the BH model cannot account for the breadth in the correlation of the rate constants for porton transfer with driving force. The origin of the discrepancy may lie in the single-mode nature of the BH model, which allows only for vibrational excitation in the low-frequency promoting mode. Excitation in the reactant and product modes of the vibration associated with the transferring proton is not taken into account in the BH model. Therefore, the discrepancy between experiment... 

Here a and are the usual oscillator creation and annihilation operators with bosonic commutation relations (73), and 0i,..., 1 ,..., 0Af) denotes a harmonic-oscillator eigenstate with a single quantum excitation in the mode n. According to Eq. (80a), the bosonic representation of the Hamiltonian (79) is given by... 

If the molecular absorption lines do not overlap within their Doppler width, excitation with single-mode lasers succeeds in populating one single rotational-vibrational level. 

Let us consider a laser oscillating at a single frequency (single-mode operation) and gas molecules inside the laser resonator which have absorption transitions at this frequency. Some of the molecules will be pumped by the laser-light into an excited state. If the relaxation processes (spontaneous emission and collisional relaxation) are slower than the excitation rate, the ground state will be partly depleted and the absorption therefore decreases with increasing laser intensity. 

The attenuation of die dipole of the repeat unit owing to thermal oscillations was modeled by treating the dipole moment as a simple harmonic oscillator tied to the motion of the repeat unit and characterized by the excitation of a single lattice mode, the mode, which describes the in-phase rotation of the repeat unit as a whole about the chain axis. This mode was shown to capture accurately the oscillatory dynamics of the net dipole moment itself, by comparison with short molecular dynamics simulations. The average amplitude is determined from the frequency of this single mode, which comes directly out of the CLD calculation ... 

A crucial part of these experiments is the preparation of the sodium atoms into the excited state by laser optical pumping. A commercial single-mode Rhodamin 6G continuous wave (cw)-dye laser (Spectra Physics model 580) is used, having 20-40-mW single mode output power when tuned to the sodium resonance line. 

Using continuous wave (cw) laser excitation it is possible to excite atoms with substantially higher efficiency than using pulsed lasers. For example a single mode laser of 1 MHz linewidth has a resolution 3 x 104 better than the pulsed laser... 

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