Gary W. Loge

Multiphoton induced fluorescence and ionization of carbon monoxide (B 1Σ

The B 1Σ+ state of carbon monoxide was created by a two‐photon absorption of 230 nm laser radiation. The B 1Σ+ state was identified by the positions of the Q branch 0–0 absorption band and the vibrational bands of B 1Σ+ → A 1Π fluorescence occurring after excitation. Collision‐induced b 3Σ+ → a 3Π emission in the 350 nm region was also observed. The lifetimes of the B 1Σ+ → A 1Π and the b 3Σ+ → a 3Π emission were obtained as well as the self‐quenching rate constants and the CO (B 1Σ)) quenching rate by N2. In addition to the two‐photon laser induced fluorescence of B 1Σ+ carbon monoxide, a three‐photon resonant (2+1) ionization was observed. Laser power dependence studies indicate that ionization of the B 1Σ+ state was nearly saturated at the laser fluence used, implying that most CO (B 1Σ+) was ionized rather than fluorescing. The measured polarization ratios for circularly to linearly polarized excitation of fluorescence and ionization was much larger than the expected values suggesting that other three...

Photodissociation dynamics of triatomic molecules

Calculations are presented for the polarization anisotropy, R = (I ‖ - I ⊥)/(I ‖ + 2I ⊥), of the fragment fluoresence (AB* →AB + hv) when a triatomic molecule (ABC) is photodissociated by a beam of linearly polarized radiation. Expressions are developed relating the polarization anisotropy to the predissociation lifetime τ of the [ABC*] complex under the assumptions that (1) the motion of the [ABC*] complex can be described by the classical motion of the prolate symmetric top, (2) the AB* excited fragment rotates in the plane of the [ABC*] complex at the time of dissociation and (3) the absorption and emission processes can be described by classical hertzian dipole oscillators. The time averaged autocorrelation function of a free symmetric top, , is used to find R(τ) for the dissociation of an [ABC*] complex in a single J, K level and for this quantity averaged over a Boltzmann distribution of J, K levels.

Dependence of diatomic photofragment fluorescence polarization on triatomic predissociation lifetime

When a triatomic molecule, ABC, is dissociated by a beam of light to yield an electronically excited diatomic molecule, AB*, the degree of linear polarization of the AB* photofragment flourescence depends in general on the lifetime of the excited parent molecule, ABC*. Analytical expressions are developed for the polarization of diatomic flourescence as a function of predissociation lifetime. These calculations are carried out under the assumptions that: (a) the absorption and emission transition dipole moments can be replaced by classical hertzian dipole oscillators; (b) the AB* fragment rotates in the plane of the ABC* parent at the time of dissociation; and (c) the motion of the ABC* parent can be approximated by the classical motion of a prolate or oblate symmetric top.

Fluorescence lifetimes and Zeeman quantum beats of single rotational levels in 3B2 carbon disulfide

The lowest observed triplet state of carbon disulfide, the R 3B2 state, was directly excited using a pulsed, narrow bandwidth dye laser (0.05 cm−1). The first reported collision‐free lifetimes of this state of CS2 were obtained, and the changes in lifetime for single rotational and vibrational levels were observed. The effect of rotation and vibration on the lifetimes indicates that the R 3B2 state has strong spin‐orbital coupling with the higher lying V 1B2 state, which gives the R 3B2 state such short lifetimes (2–5 μs) for a triplet state. A magnetic field was observed to have no effect on the lifetimes, which is consistent with this strong spin‐orbital coupling scheme. Zeeman quantum beats were observed on the fluoresence decay curves in a magnetic field. Beat frequencies were obtained as a function of magnetic field strength to obtain permanent magnetic dipole moments for the R 3B2 state vibrational levels. The magnetic dipole moments were found to be independent of rotational angular momentum (N dep...

Lifetime of the A 2Σ+, v’=0 level of HS measured using the Hanle effect

H2S was photodissociated using an ArF excimer laser at 193 nm to form HS photofragments. LIF spectra of HS in the region of 324 nm were obtained using a pressure tuned dye laser with improved resolution of the R1 and RQ21 branches in the A 2Σ+←X 2Π3/2 transition. The zero field linewidths and the Zeeman splitting of the RQ21 (1.5) line in a magnetic field were obtained. The latter was used to verify the g values expected for a Hund’s case (b) 2Σ+ upper state and Hund’s case (a) 2Π lower state. Depolarization of fluorescence in a magnetic field using the RQ21 (3.5) line with σ‐polarized LIF excitation was used to determine an estimated lifetime of 0.15–0.3 ns for the lowest vibrational level of the 2Σ+ state. A lower limit on the lifetime of 0.17 ns was determined from the measured adsorption linewidths. No rotational dependence on the linewidth was observed.

Polarized photofragment emission following two‐photon dissociation of XeF2 at 266 nm

The polarization anisotropy of molecular photofragment emission following two‐photon induced dissociation of triatomic molecules is calculated in the high J limit. This theoretical treatment is applied to the two‐photon induced photodissociation of XeF2 at 266 nm from which the XeF B→X emission displays a polarization anisotropy of 3.5%±1.5%. The XeF2 dissociating state is shown to be non‐totally symmetric by measurement of the relative intensities of B→X emission excited by linearly and circularly polarized actinic radiation. Dissociation proceeds via a 1πu Rydberg state.

Two‐photon pumped CO B‐A laser

An optically pumped CO laser that involves a two‐photon excitation of the B state is described. A photon conversion efficiency of 15% is determined for the 0‐1 and 0‐2 vibraonic transitions of the B‐A system. It appears that the major portion of the emitted light is the result of amplified spontaneous emission due to the high gain of the transition.

A quantum mechanical calculation of photofragment absorption or emission polarization

The polarization of light emitted by excited state diatomic photofragments produced using linear polarized radiation to dissociate a triatomic molecule is calculated using a quantum mechanical treatment of molecular rotational angular momentum and assuming direct dissociation. The photofragment emission polarization is found to depend upon the relative orbital angular momentum of the separating fragments as well as the individual angular momenta of the parent and fragment molecules.

DIRECT OBSERVATION OF TRANSIENT FLUORINE ATOMS WITH 25-MU M WAVELENGTH-STABILIZED DIODE LASER ABSORPTION

Through the use of continuous diode laser absorption, detection of transient fluorine atoms with an initial number density in the range of 10(14) cm(-3) has been demonstrated. A crucial part of the continuous-detection technique was laser frequency stabilization with a reference cell of atomic fluorine with Zeeman modulation of the absorption lines to generate a feedback signal. Long-term wavelength stability was demonstrated with second-harmonic phase-sensitive detection of the second-derivative signal for periods up to several hours. For determination of the short-term wavelength stability in the range of microseconds to seconds, a transient signal was generated by photolysis of F(2) with an excimer laser at 308 nm. The initial diode laser absorption was compared to a calculated value obtained from the measured excimer laser fluence, the known dissociation cross section of F(2), and the atomic fluorine absorption cross section, which included a statistical population distribution, the finite bandwidth of the laser dode, and the effects of pressure broadening. The observed absorption was approximately 33% less than the calculated value, possibly because of the diode lasers wavelength instability on the time scale of a few seconds, which is consistent with an observed amplitude instability from pulse to pulse when pulsed at 1-10 Hz.

Detection of transient fluorine atoms

A KrF laser with a fluence of 50 mJ/cm2 was used to photolyze either uranium hexafluoride or molecular fluorine, yielding a transient number density of fluorine atoms. The rise and decay of the atomic fluorine density was observed by transient absorption of a 25‐μm Pb‐salt diode laser. To prevent the diode laser wavelength from drifting out of resonance with the atomic fluorine line, part of the beam was split off and sent through a microwave discharge fluorine atom cell. This allowed a wavelength modulation‐feedback technique to be used to lock the diode laser wavelength onto the atomic line. The remaining diode laser beam was made collinear with the excimer laser beam using a LiF window with a 45° angle of incidence to reflect the infrared beam while transmitting most of the uv beam. Using this setup along with a transient digitizer to average between 100 and 200 transient absorption profiles, fluorine atom number densities on the order of 1014 cm−3 in a 1.7 m pathlength were detected. The signal observ...