LASER INDUCED FLUORESCENCE OF HCN FROM THE A~1A′′\tilde{A} {^{1}}A'' STATE

Author Institution:Fluorescence spectra of the near vaccum ultraviolet $\tilde{A} ^{1} A^{\prime\prime} \rightarrow \tilde{X} ^{1}\Sigma^{+}$ transition of HCN have been recorded and assigned. A tuned (0.15A bandwidth) ArF excimer lasser is used to excite the (010)$^{1}$, (000)$^{2}$ and (000)$^{0}$ individual vibronic features in the $\tilde{A} ^{1} A^{\prime\prime}$ state. Fluorescence terminating on high vibrational levels of the $\tilde{X}^{1}\Sigma^{+}$ state dominates the emission spectra. Progressions in the $v_{2}$ bending and $v_{3}$ stretching modes are assigned. The fluorescence spectra, emission assignments, fluorescence lifetimes and the fluorescence excitation spectrum will be reported and discussed

LIFETIMES OF THE \~{B} STATE OF CH3ICH_{3} I AND CD3ICD_{3} I USING FEMTOSECOND RESONANCE ENHANCED MULTIPHOTON IONIZATION

Author Institution: Code 6111, Chemistry Division, Naval Research LaboratoryFemtosecond pulses in the deep uv (near 200 nm) have been used to directly measure The $\widetilde{B}$ state lifetimes for gas phase $CH_{3}I$ and $CD_{3}I$ by time-resolved MPI. Tunable deep uv femtosecond excitation pulses were generated by sum frequency generation in BBO of a frequency doubled CPM laser with amplified portions of continuum. For the vibrationless state of $CH_{3}I (CD_{3}I)$ the lifetime is 1.38 ps (1.84 psec). It is longer for the $\nu_{3}=1$ level of $CH_{3}I$ (1.80 ps). These values indicate the multidimensional nature of the dissociation and their isotopic and mode dependence will be compared with those obtained by indirect measurements

RADIATIVE AND RADIATIONLESS TRANSITIONS IN OXALYL FLUORIDE RESULTING FROM SINGLE VIBRONIC LEVEL EXCITATION

A. Baronavski has been an NRC-NRL Postdoctoral Associate.Author Institution: Naval Research LaboratorySingle vibronic level excitation of oxalyl fluoride $(C_{2}O_{2}F_{2})$ near the origin $(\lambda \geq 3000${\AA}) of the first excited singlet state by a tunable frequency doubled dye laser gives rise to fluorescence. The zero-pressure fluorescence lifetime of these levels $(\sim 22 \mu ~sec)$ is probably purely radiative. The bimolecular self-quenching rate constants for these radiative levels are on the order of $20$ x $10^{6} sec^{-1} Torr^{-1}$. This rate, which is dominated by collision-induced intersystem crossing, gives rise to intense phosphorescence with a lifetime of several milliseconds. Strong phosphorescence is still evident at pressures below 10 mTorr. Scanning of the fluorescence excitation spectrum demonstrates that only levels near the origin result in fluorescence. The unimolecular radiationless process probably corresponds to the onset of predissociation giving two FCO radicals in the primary photochemical process. The energy dependence of this process has been mapped out. Experimental details and the radiative and radiationless processes will be discussed

Ultraviolet photodissociation dynamics of H2S and D2S

Nascent SH(X2 Πi, v = 0,1) and SD(X2 Πi, v = 0,1) rotational state population distributions, spin-orbit state population ratios, and A-doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ = 193, 222, and 248 nm) and D2S (λ = 193 and 222 nm), respectively. Nascent SH(X2 Πi, v = 0) rotational state distributions following 193 nm photodissociation of cold H2 S in a free jet expansion vs 300 K H2 S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X2 Πi, v = 1) and SD (X2 Πi, v = 1) have been recorded for the first time. Rotational state distributions for SH(X2 Πi, v ) and SD(X2 Πi, v ) are independent of v . A-doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consistent with an impact parameter model for the dissociating triatomic molecule

APPLICATION OF SATURATION SPECTROSCOPE FOR MEASUREMENT OF Na AND MgO CONCENTRATIONS IN ACETYLENE FLAMES

Louise Pasternak has been an NRC/NRL Resident Research Associate.Author Institution: Naval Research LaboratoryAbsolute atomic Na and MgO concentrations in an acetylene air flame are determined using laser-induced fluorescence saturation spectroscopy. The combustor is a slot burner using salt solutions aspirated into premixed acetylene and air to create a stable, homogeneous, atmospheric pressure flame. Independent atomic and molecular absorption measurements are made for Na and the MgO $(X^{1}\Sigma^{+})$ state which are in agreement with the saturation spectroscopy concentration determinations. We will report laser-induced fluorescence spectra of MgO $(B^{1}\Sigma^{+} - X^{1}\Sigma^{+})$ and $(B^{1}\Sigma^{+} - A^{1}\Pi)$ transitions. From the relative intensities of these spectra it is apparent that a significant inversion exists between MgO A and X states in the flame

ArF EXCIMER LASER PHOTODISSOCIATION OF NH3NH_{3}: INTERNAL ENERGY DISTRIBUTION IN NH2NH_{2} A2AA_{2}A, AND X2B1X_{2}B_{1}

$^{1}$ H. Okabe and M. Lenzi, J. Chem. Phys. 47, 5241 (1967). $^{2}$ J. Masamet, A. Gilles, and C. Vermeil, J. Photochem., 3, 417 (1974/1975). $^{3}$ K. Dressler and D. A. Ramsay, Phil. Trans. Roy. Soc. London 251, 553 (1959). $^{4}$ J.W.C. Johns, D. A. Ramsay, Can. J. phys. 54, 1804 (1976). V. M. Donnelly has been a NRC/NRL Resident Research Associate.""Author Institution: Naval Research LaboratoryLow pressure ($P < 35$ mtorr) samples of $NH_{3}$ are photolyzed with a Tachisto Model XR 150 pulsed excimer laser operating on the 193 nm line of ArF (20 nsec pulse duration, 35 mJ per pulse, $\sim 3$ Hz repetition rate). Contrary to previous $^findings^{1,2}$ (for $NH_{3}$ dissociation) in this excitation region, we find that $NH_{2} (A^{2}A_{1})$ is a major product formed by single photon, primary photolysis. Strong $NH_{2} (A^{2} A_{1} \rightarrow X^{2}B_{1})$ banded emission is observed between 620 and 1100 nm. Most of the lines are assignable to transitions catalogued in absorption by Dressier and $Ramsay^{3}$ and Johns, Ramsay, and $Ross^{4}$ The relative intensities in our emission experiments are much different than those in absorption, indicating that much of the 0.74 eV excess dissociation energy appears as bending vibrational energy and rotational excitation about the a-axis. These observations are expected, an the basis of changes in geometry in the primary photolysis process. Experiments are also under way to probe energy distributions with the $NH_{2} (X^{2}B_{1})$ primary photofragment, Using dye laser-induced fluorescence excitation spectroscopy on the $A^{2}A_{1}\leftarrow X_{2} B_{1}$ transition. In addition, $NH(A^{3}\Pi)$ is formed in a two photon $NH_{3}$ dissociation process, giving rise to a $A^{3} \Pi\rightarrow X^{3} \Sigma^{-}$ emission at 336 nm