Harold K. Haugen

Application of an InGaAsP diode laser to probe photodissociation dynamics: I* quantum yields from n‐ and i‐C3F7I and CH3I by laser gain vs absorption spectroscopy

A room temperature heterostructure InGaAsP laser diode operating at 1315 nm is employed for the first time for detection of I*(2P1/2) and I(2P3/2) atoms. The cw diode probe laser is used to study I* yields in the photodissociation of n‐ and i‐C3F7I and CH3I by the new technique of time‐resolved laser gain vs absorption spectroscopy. Preliminary quantum yields determined at 266 nm for n‐C3F7I, i‐C3F7I, and CH3I are 102±4%, 102±7%, and 73±4%, respectively. With further refinements to the diode laser set‐up, highly accurate quantum yields will be possible.A room temperature heterostructure InGaAsP laser diode operating at 1315 nm is employed for the first time for detection of I*(2P1/2) and I(2P3/2) atoms. The cw diode probe laser is used to study I* yields in the photodissociation of n‐ and i‐C3F7I and CH3I by the new technique of time‐resolved laser gain vs absorption spectroscopy. Preliminary quantum yields determined at 266 nm for n‐C3F7I, i‐C3F7I, and CH3I are 102±4%, 102±7%, and 73±4%, respectively. With further refinements to the diode laser set‐up, highly accurate quantum yields will be possible.

Accurate quantum yields by laser gain vs absorption spectroscopy: Investigation of Br/Br∗ channels in photofragmentation of Br2 and IBr

A two‐laser pulse‐and‐probe technique is used to study photofragmentation of Br2 and IBr over the wavelength range 450–530 nm. The metastable Br(2P1/2–2P3/2) transition is probed by time–resolved laser gain vs absorption spectroscopy using a tunable color center laser. This new approach to the measurement of quantum yields provides highly accurate absolute values for Br*(2P1/2) production. The peak quantum yield for Br2 photodissociation is φ=87% at λ=500 nm. The difference between the spectral variation of φ and the total absorption spectrum characterizes the A state of bromine, which contributes ≲14% to the absorption spectrum at λ=510 nm. The peak in the Br* yield from photofragmentation of IBr is φ∼73% at λ=500 nm. The present absolute IBr data together with the previous molecular beam studies suggest a reassessment of the contributions of the continuum states in IBr. The laser gain vs absorption method for obtaining quantum yields is readily generalized to other atoms and molecules.

Infrared double resonance spectroscopy of V‐T, R relaxation of HF(v=1): Direct measurement of the high‐J populations

The V‐T, R relaxation of HF(v=1) by HF is studied by infrared pulse‐probe transient absorption measurements using a tunable F‐center laser. It is found that a substantial fraction of the relaxation occurs through the high‐lying rotational levels of v=0. The results indicate that the states J=10–14 comprise ∼20%–40% of the population in the total relaxation, with the distribution increasing rapidly with decreasing J in this range. The observed signals, corrected for cascade, correspond to the fractions: J=10, ∼10%–40%; J=11, ∼4%–10%; J=12, ∼2%–6%; J=13, ∼1%–2%; and J=14, ∼0.05%–0.35%. In addition, the V‐T, R rate determined here, k=1.46±0.15×10−12 cm3 molecule−1 s−1 reconfirms previous double resonance results. Also, studies involving buffer gas addition (Ar) give no indication of ‘‘bottleneck’’ effects on the high J’s. In fact, the R–R, T rates always exceed the V‐T, R rate by one to two orders of magnitude. For J=11–13, the R–R, T rates range from ∼1.2×10−10 to 6×10−11 cm3 molecule−1 s−1.

Effect of spin—orbit excitation on chemical reactivity: Laser transient absorption spectroscopy of Br(2P12, 2P32 + IBr reactive dynamics

Abstract A laser pulse-and-probe technique incorporating a tunable infrared color center laser is used to study the reactions and quenching of Br( 2 P 1 2 , 2 P 3 2 ) with IBr. A highly selective spin—orbit effect on chemical reactivity is observed. The ground-state reaction, Br( 2 P 3 2 )+IBr → Br 2 + 1, k = (4.6 ± 0.6)×10 −11 cm 3 molecule −1 s −1 proceeds at a rate ≥ 40 times faster than the rate of total Br- quenching and reaction with IBr.

Collision‐induced dissociation of laser‐excited Br2[B 3Π(0+u);v’, J’]: Formation of Br*(2P1/2)+Br(2P3/2) at energies 1–5 kT below dissociation

Absolute rate constants are measured for the dissociation of initially excited Br2[B 3Π(0+u);v’,J’] into Br*(2P1/2)+Br(2P3/2) fragments by thermal energy collisions with Br2(X 1Σ+g), Xe, and Ar at 297 K. A 0.04 cm−1 etalon‐narrowed pulsed dye laser populates specific rovibrational levels of isotopic Br2 which are 1–5 kT below the B‐state dissociation limit; in addition the laser directly photodissociates molecules which are in thermally excited vibrational levels. The method used to determine the absolute rate constants combines four sets of experimental determinations, which include infrared detection of Br*, visible Br2(B) fluorescence lifetimes, absorption spectroscopy of Br2 (B←X), and transient gain‐vs‐absorption spectroscopy on Br/Br*. At 1 kT below the dissociation limit, the absolute rate constants for collisional dissociation to Br*+Br are 2.9×10−10, 1.2×10−10, and 5.4×10−11 cm3 molecule−1 s−1 for Br2, Xe, and Ar, respectively. The results represent an average of some rotational and vibrational s...

Laser determinations of ‘‘hot band’’ quantum yields: Br*(2P1/2) formation in the continuum absorption of Br2 at 510–550 nm

Absolute quantum yields of Br* in the photodissociation of thermally excited vibrational levels of Br2 are measured by transient gain vs absorption probing of the Br*(2P1/2)–Br(2P3/2) transition at 2.7μm with an F‐center laser. An etalon‐narrowed pulsed dye laser with a linewidth of 0.04 cm−1 is used to excite continuum regions between the bound Br2(B–X) transitions of isotopic Br2(∼81% 81Br2) at selected wavelengths between 510 and 550 nm. The threshold wavelength for production of Br*+Br from v″=0, J″=0 is 510.7 nm. At 297 K the Br* yields vary from 85% at 510 nm to 12% at 550 nm. Absorption coefficient data measured with the laser together with the Br* quantum yields at both 297 and 373 K enable a direct determination of the B‐state continuum absorption. The data are used to determine the individual contributions of the B 3Π(0+u), 1Π(1u) and A 3Π(1u) states to the continuum absorption in this spectral region. Good agreement is obtained between the measured B‐state absorption coefficients and those calc...

Application of semiconductor diode lasers to probe photodissociation dynamics

Tunable diode lasers are rapidly proving to be useful probes of atomic and molecular species. In the present study we utilize a semiconductor diode laser to probe the photodissociation dynamics of alkyl iodides. A room temperature InGaAsP diode laser operating at 1315 nm is used to probe the transition between I*(2P1/2) and I(2P3/2) atoms. I* quantum yields are obtained in the UV laser photolysis of n− and i−C3F7I, CH3I and ICN by time resolved laser gain versus absorption spectroscopy. The high amplitude stability of the diode laser and the internal normalization of the gain versus absorption technique allow for a sensitive and accurate determination of the I* quantum yields.

Linear and nonlinear spectroscopy at a cesium vapour/dielectric interface using a semiconductor laser

Abstract Linear and nonlinear spectroscopy on the Cs (6s2S 1 2 -6p2P 3 2 ) transition at 852 nm is performed at a cesium vapour/dielectric interface. Two reflection phenomena are studied: selective reflection in a linear laser-atom interaction regime, and saturation spectroscopy in the evanescent wave. The spectral narrowing of selective reflection is observed as a function of angle in the range from near-normal incidence to the critical angle. The substantial narrowing which is observed for perpendicular incidence is found to decrease monotonically away from the surface normal. Studies of saturation spectroscopy beyond the critical angle demonstrate sensitive Doppler-free detection of near-surface atoms.