LASER Q switched

Nd YAG Q-switched laser, in fine art examination/ conservation, 11 413 N-donating ligands, complexation with uranium, 25 434-436 Near-edge X-ray absorption fine structure (NEXAFS), 24 72... 

Cyanines have been widely used as laser dyes, and as saturable absorbers in modelocked and Q-switched laser systems. 8, 50) The propensity of most cyanines to photooxidize which makes them useful in photographic film and as saturable absorbers makes them less than desirable as fluorophores in other applications. The use of... 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

A second method used to examine the same equilibria is the laser photoperturbation technique. Irradiation with a Q-switched laser pulse at 1060 nm depletes the tetrahedral isomer irradiation at the doubled frequency of 530 nm depletes the planar isomer. In both cases the same relaxation time of 0.93(4) jisec is observed for reestablishment of the equilibrium. 

Figure 7.13 Shape of the pump flash light spectrum and of the Q-switched laser light, t, time I, light intensity...

Lasers have been used to initiate deton in RDX. Three types of initiation mechanisms have been described (Ref 102) (1) instantaneous deton caused by a shock wave in a thin metallic film (deposited on the expl) with the shock wave generated by a Q-switched laser pulse (2) instantaneous deton by direct interaction of a Q switched laser pulse and the test expl and (3) DDT produced by free-running laser pulses. Coarse RDX cannot be initiated, but milled RDX (particle size less than 40 microns) is readily initiated at various packing densities. The threshold fluences for the initiation of 1.18g/cc l,52g/cc milled RDX via mechanism (1) are 45,3J/cm2 and 127.9J/cm2, respectively. Detons are either essentially instantaneous or the sample bums without deton. For direct initiation [mechanism (2)], the threshold laser energy for 1.18g/cc RDX was 0.8J, or the same as in thin film initiation. However, deton was no longer instantaneous but required about 2 microsec for build-up. The 1.52g/cc RDX was initiated directly without delay (laser energy not given)... 

The molecular hyperpolarizabilities are / , 7, and a is the molecular polarizability. Typical values of / are 10 30 esu (esu units mean that the dimensions are in CGS units and the charge is in electrostatic units, thus / in esu means / in units of cmzesuz /erg2) [1-4]. For an electric field typical of Q-switched laser light, 104 statvolts/cm, the contribution to - //(0) from /3S2 is 10 4 debye. These polarizations are infinitesimal on the scale of our usual chemical thinking. Yet, these small polarizations are responsible for the exotic effects described throughout this volume. The perturbation theory approach used to describe these properties is justified by the fact that so little charge actually moves. 

Bradley Moore andc o-workers167 have ingeniously applied vibrational lasers to the study of V-T and especially V-V energy transfer. The principle is to expose a gas sample to a Q-switched laser pulse and to monitor the infrared fluorescence... 

Q-switching) laser provides pulses that have 7-9 ns width, 100 MW peak power and an optimum repetition rate of 20 Hz. 

In a Q-switched laser, the population inversion, usually produced in the same way as CW operation, builds up by making the cavity quality factor Q unfavorable for lasing. Then, when the pump energy stored in the laser medium is at the desired level, the Q is suddenly adjusted (electro- or acousto-optically) to release the pulse. This results in high peak powers. 

Q-switched laser A laser in which the state of the device introducing important losses in the resonant cavity and preventing lasing operation is suddenly switched to a state where the device introduces very low losses. This increases rapidly the Quality factor of the cavity, allowing the build-up of a short and very intense laser pulse. Typical pulse durations are in the ns range. The Q-switching may be active (a rotating mirror or electro-optic device) or passive (a saturable absorber). 

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

Figure 2. Schematic layout of a typical suprananosccond kinetic spectrometer Q Q-switched laser with harmonic generators L lens X cw monitoring lamp S shutter A aperture (see Figure 3 for detail) C sample cuvette M monochromator PM, PD photodetectors O digital oscilloscope PC personal computer.

The photoisomerization is induced by irradiation with green light (532 nm) from either a continuous laser or pulsed laser (frequency doubled Nd.-YAG Q-switched laser operated in single pulse mode). The second harmonic is generated by a pulsed optical source such as produced by NdrYLF (1053 nm) or Nd YAG (1064 nm) Q-switched nanosecond laser. The probe... 

Lentigines and keratoses should be treated by another method in parallel with AHA peels. There are other, more efficient, techniques to treat these problems liquid nitrogen, dry ice, Only Touch peel, intense pulsed light (IPL), laser, etc. Dr Robert Vergereau (France) compared the use of dry ice, Erbium laser, Q-switched laser, coagulation and Only Touch peel. He concluded that If all these methods are satisfactory, in my opinion, the technique using trichloroacetic acid is the most beneficial . 

---