William K. Bischel

Detection of fluorescence from O and N atoms induced by two-photon absorption

The 3p (3)P state of O and the 2s(2)2p(2)3p (4)D(0) states of N are populated by two-photon absorption at 226 and 211 nm, respectively, and the resulting near-IR fluorescence is detected. The exciting photons are provided by stimulated Raman frequency shifting, and the experiments are performed in a flow discharge. The measured lifetime of 39 (0) and 27 (N) nsec and quenching rate constants of 2.5 x 10(-10) cm(-3) sec(-1) for collisions of N(2) with each atom indicate promise for this method as a diagnostic tool in flames and plasmas.

Two-photon laser-induced fluorescence in oxygen and nitrogen atoms

Abstract Two-photon absorption is used to populate the 3p 3 P state of O and the 2s 2 2p 2 3p 4 D o state of N in a flow discharge. Exciting photons are produced by anti-Stokes Raman frequency conversion of tunable UV laser radiation; the resulting near IR fluorescence from the excited state yielded lifetimes, quenching rates, and relative two-photon transition probabilities.

Wavelength dependence of the absolute Raman gain coefficient for the Q(1) transition in H 2

The absolute Raman gain for the Q(1) vibrational Raman transition in H2 at 4155 cm−1 has been measured at pump (Stokes) wavelengths of 532 (683), 477 (594), and 307 (351) nm using a pulsed pump and a cw probe laser, both of which were single frequency. Good agreement is found between the measured and calculated Raman steady-state gain coefficients. Empirical formulas are derived to calculate the Raman gain as a function of wavelength, density, and temperature.

Nonlinear optical processes in atoms and molecules using rare-gas halide lasers

The use of nonlinear optical processes expands the flexibility of excimer systems in the study of a wide range of atomic and molecular phenomena and materials. These mechanisms have already allowed for the selective excitation of states in the 10 to 20 eV range involving bound state excitation, ionization, and molecular dissociation. Specific examples involving the electronic excitation of H 2 , Kr, and Xe, the production of Xe+for the analysis of the molecular properties of XeF*, and nonlinear photodissociation of N 2 O and OCS are discussed.

Nascent NO vibrational distribution from 2485 Å NO2 photodissociation

The initial NO vibrational level distribution has been determined for NO2 photodissociation at 2485 A. Excitation spectra of the NO vibrational levels were measured by using both the NO A 2Σ+←X 2Π and B 2Π←X 2Π transitions, the latter being somewhat stronger due to saturation effects. It was determined that the NO population was strongly inverted, with most of the nascent NO being in v=6–8; the thermodynamic limit is v=8. Injection locking of the KrF laser output permitted study of the 2491 A NO2 band, and it was evident that the increased absorption in this region gave greatly enhanced signal levels in the excitation spectra, at those wavelengths where NO2 and NO absorption lines coincide. It was demonstrated that in the 2640–2850 A wavelength region, NO2 can be detected by use of a single dye laser, simultaneously dissociating NO2 and electronically exciting the resultant vibrationally hot NO. Deactivation of NO(v=8) by NO2 was found to proceed with a rate coefficient of 1.1×10−11 cm3 molecule−1 s−1, wh...

A new blue‐green excimer laser in XeF

A new blue‐green excimer laser has been demonstrated on the C‐A transition in XeF, with an output energy of greater than 1 mJ. Xe*2 excimer fluorescence at 172 nm was used to photodissociate XeF2, producing XeF[B (1/2)] and XeF[C (3/2)]. The B (1/2) state was collisionally relaxed to the C (3/2) state with an Ar buffer. Lasing then occurs sequentially on the B (1/2) ‐X (1/2) and C (3/2) ‐A (3/2) transitions. Measurements of the C‐A laser spectrum showed a peak wavelength of 483 nm with a bandwidth of 12 nm. This new laser is potentially highly efficient and scalable and should be tunable over a bandwidth greater than 40 nm.

Photolytically pumped XeF (C→A) laser studies

Properties of the visible XeF (C→A) photolytically pumped laser system have been investigated in detail. The research covers four main areas: (1) absolute fluorescence intensities from the C and B states, (2) C→A fluorescence spectra as a function of pressure and buffer gas, (3) gain/absorption measurements from 400–650 nm, and (4) demonstration of tunable laser operation over the 454–525‐nm range. All results reported here indicate that the XeF (C→A) laser could be a highly efficient visible laser system.

Laser generation from 6 to 35 μm following two‐photon excitation of ammonia

Laser action has been observed for the first time in ammonia subsequent to two‐photon absorption. Oscillation has been demonstrated in 14NH3 following two‐photon excitation by a pair of CO2 TEA lasers operating on the P (34) and P (18) lines in the 10.4‐μm band, respectively. The observed wavelengths are 6.27, 6.69, 12.11, 13.72, 15.88, 15.95, 18.92, 19.55, 26.10, and 35.50 μm. Identification with existing spectroscopic information indicates that eight of the laser transitions occur within the ν2 manifold and originate by radiative pathways from the 2ν2−(5,4) level at 2115.88 cm−1, whereas the remaining two, at 6.27 and 6.69 μm, take place in the ν4 mode. The usefulness of employing such lasers as sensitive probes for molecular collisional processes in excited vibrational levels is indicated.

Energy ordering of the excited states of XeF

Ar/Xe/NF3 mixtures were excited by the focused beam from an ArF (193 nm) laser. Xe+ ions are produced by two‐photon ionization, the electrons attach to make F−, and the ions recombine to make XeF*. Radiation is observed in the XeF(B 1/2) →XeF(X 1/2) bands near 351 nm and in the broader XeF(C 3/2) →XeF(A 3/2) band near 460 nm. At low background gas pressure, mostly B‐X uv emission is observed. As the argon pressure is increased to 1000 Torr, the visible/uv band intensity ratio increases to about 3 to 1. We conclude from these results that the C (3/2) state lies 700±70 cm−1 below the B (1/2) state. This conclusion should have a significant impact on our understanding of the fluorescence yields and laser performance of e‐beam‐excited XeF.

Absolute calibration of a fluorescence collection system by Raman scattering in H2

A new method for absolute calibration of the detection efficiency of fluorescence collection systems is described for wavelengths covering the 180–2000-nm range. The radiation source for the technique is spontaneous Raman scattering in H2. An analytic expression is derived for the scattering cross section averaged over the solid angle of the fluorescence collection system. From this result, an optical system is calibrated at 845 nm, demonstrating that the technique is easy to use, is quick to implement, and gives accurate results.