10911
J. Phys. Chem. 1993,97, 10911-10919
Photofragment Vector Correlations Measured by Transient Absorption Spectroscopy: CN Fragments from Ethyl Thiocyanate Photodissociation Gregory E. Hall' and Ming Wu Chemistry Department, Brookhaven National Laboratory, Upton, New York 1I973 Received: June 29, 1993; In Final Form: August 13, 1993'
The correlated angular distribution of translational and rotational motion of photofragments is probed by Doppler-broadened transient absorption line shapes. The one-photon nature of the probe process reduces the complexity of the analysis, while maintaining sensitivity to five bipolar moments of the translational and rotational angular distributions. The 193-nm photodissociation of C2HsSCN illustrates the method, with C N products detected with a titaniumsapphire ring laser probing the A211-X22+ transition. The vector correlations in the high rotational states of C N indicate a direct dissociation following a transition of mixed symmetry a t 193 nm: the recoil is preferentially but not exclusively parallel to a transition moment, and the perpendicular component is dominated by a rotation axis parallel to a C2H5SCN transition moment. The strong rotational excitation is generated by planar bending forces in a bent excited state. Lower rotational states are formed with a composite kinetic energy distribution, indicating an additional, slower, less highly polarized channel that partitions more energy into the unobserved QH5S radical than does the direct channel. At 248 nm, a weaker absorption also generates C N photofragments with a preferentially parallel recoil, requiring a reassessment of the excitedstate assignments in the alkyl thiocyanates.
Introduction The measurement and interpretation of the correlated angular distributions of photofragment velocities and angular momenta have led to a new level of microscopic detail in the characterization of molecular photodi~sociation.~-5Excited-state symmetriesand lifetimes, transition-state and exit channel geometries, and multiple surface effects can frequently be understood with the aid of vector correlation measurements, when electronic spectra and product-state distributions alone may be inconclusive. Previous measurements have used sub-Doppler laser-induced fluorescence (LIF) spectroscopywith oneG9or more1C-12excitation photons, or resonant ionization spectroscopy without sub-Doppler excitation, but with velocity resolution in one dimension by timeof-flight13or in twodimensions by imaging the ion distribution.14J5 The theoretical frameworkfor one-photon LIF presented in 1986 by Dixon16 included the simpler case of sub-Doppler line shape analysis of spontaneous fluorescence from excited-state photofragments. No examples of this type of experiment have been reported, to our knowledge, although it should be noted that Hatano and co-workers17had much earlier observed that Doppler line shapes of excited hydrogen atom emission following electron impact dissociation of molecules can depend on polarized m distributions, correlated to the recoil velocity. High-resolution transient absorption of photoproducts is not an uncommon technique,la20 yet to obtain adequate sensitivity,a long absorption path length is generally obtainedby means of parallel propagation of photolysis and probe beams, which severely limits the range of experimental geometries. Cline et al.20 used a transverse multipass geometry to investigate anisotropicvelocity relaxation following photodissociation of C3F71, although they deliberately probed the J = 1 1 2 stateof atomiciodine, which cannot bealigned. We report the use of a CW Tisapphire ring laser and a transverse multipass flow cell to characterize the vector correlations in the CN product of the photodissociation of C2HsSCN at 193and 248 nm. Adjustment of polarization optics in a single orthogonal beam geometry permits complete access to the twodimensional space of angles between the linearly polarized dissociation axis and the probe propagation and polarization
* To whom correspondence should be addressed.
* Abstract published in Advance ACS Absrracu, September 15, 1993. 0022-3654/93/2097-109 11$04.00/0
directions. The nascent Doppler-broadened line shapes measured for different polarizations and rotational branches depend on only five low-order bipolar moments of the joint velocity and angular momentum distribution whose values can be extracted simply and without unfounded approximation. The photodissociation of thiocyanates and isothiocyanateshas been previously investigated by flash photolysis2' and chemiluminescenceZ2to detect NCS fragments in ground and excited states, respectively, and in this group by laser-induced fluorescence23 to detect CN and NCS in their ground states. The nearUV absorption spectra of these compounds all resemble the corresponding spectra of SCN- and HNCS, where the low-lying singlet excited states have been a~signed2~Js as lZ- and lA at 248 and 193 nm. Ethyl thiocyanate dissociates by two pathways at both 193 and 248 nm:
+ hv C2HsSCN + hv
C2H,SCN
-
+ CN(X,2Z+) C2H5 + SCN(X,ZII)
C2H,S
-
Following 193-nm dissociation, the CN channel is characterized by a broad rotational distribution in vibrational levels up to u = 6, as well as a sharply peaked rotational distribution at very high N (70 f 10 for v = 0). The comparison with HNCS and CzH5NC.S photodissociations, which surprisingly also produced CN photo fragment^?^ suggested excited-state isomerization prior to dissociation. The use of vector correlation analysis in combination with the state distributions for these multicomponent fragmentations can clarify the excited states involved, identify the sourceof rotational excitation, look for further signs of excitedstate isomerization, and measure the internal energy deposited in the unprobed coincident fragments. A major goal of these experimentswas to compare the three-dimensionalsignatures of the dissociation dynamics from the related isomeric precursors RSCN and RNCS. Unfortunately, we have not been able to detect CN photofragments from either C ~ H S N C S or HNCS following 193-nm dissociation in our current apparatus, in apparent contradiction to earlier work.23 We now believe the quantum yield for CN production from the isothiocyanatesto be very small, if it is formed at all. The present report describes the 0 1993 American Chemical Society
Hall and Wu
10912 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
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Figure 1. Experimental schematic. The pulsed excimer laser is polarized by Brewster stack polarizer PB;the CW probe Tksapphire laser has its polarization and intensity controlled by Px,a Babinet-Solielcompensator and Glan polarizer. Transmittedsampleand referencebeams are detected in photodiodes D1 and DO.
Figure 2. Excitation-detection geometry for transient absorption of photoproducts. The linearly polarized dissociation laser has an electric vector a along Z. The Doppler-resolvingprobe laser propagates along Z',at an angle 0. from the Z axis in the XZ plane. The polarization of the analysis laser e, has an azimuthal angle x, around the Z'direction, measured from the XZ plane. The probe polarization angle, xaris of no consequence when 6, = 0.
apparatus and analysis for extracting vector correlation information from transient absorption data. Measurements of the C2H~SCN dissociation at 193 and 248 nm show distinct dynamics that can be related to the different excited states involved.
Experimental Section Transient absorption spectra of CN photofragments have been observed in a dual-beam apparatus depicted in Figure 1. Pulsed excitation light from an excimer laser (Questek 2240, unstable resonator) made a double pass through the sample cell. The 193-nmArF (or 248-nm KrF) excimer beam was polarized with a stack of eight thin, fused silica Brewster plates, providing about 95% linear polarization, which could be selected at an angle of ea from the plane containing the multiple reflections of the probe beam. Typical polarized 193-nm pulse energies entering the cell were 5-10 mJ in an 8-mm-diameter beam at 10 pulses/s. KrF excimer light at 248 nm had a pulse energy several times higher, but otherwise the same. Ethyl thiocyanate vapor was metered into the sample cell at 0.5 sccm with a mass flow controller (MKS 1047B) and pumped with a throttled 4-in. diffusion pump to maintain a 50 mTorr pressure, as monitored by a capacitance manometer. Ethyl thiocyanate and ethyl isothiocyanate were obtained from Aldrich and degassed before using. HNCS was prepared by the method of Durig and Wertz,26heating a mixture of KSCN and KHS04 under vacuum, trapping the gaseous products in a liquid nitrogen trap, and separating the HNCS from OCS and HCN impurities by pumping through a trap held at -40 OC. The identity of the purified HNCS was verified by infrared spectra. The high-resolution CW probe was a single-mode (
