A novel experimental technique to measure the energy- dependent unimolecular dissociation rate k(E) of radical species is presented. Internally excited CH₃CO radicals were formed by ultraviolet photodissociation of CH₃COCl, and the subsequent decay of these radicals was detected by subpicosecond time-clocked photofragment imaging. The CH₃CO radicals with different internal energies were dispersed in space by their recoil velocities, and their decay rates were measured for each internal energy.

A simple, cost-effective method has been developed to investigate the quantum-state correlation of the two fragments and the vector properties of a photofragmentation process. The method combines the commonly used state- selective REMPI detection technique with the core-sampling concept in high resolution ion velocity measurements. To demonstrate the merits of this approach and to explore the new dynamical information obtained from these correlation measurements, two examples involving O(³P_j) fragments are presented for illustrations.

Photodissociation of OClO at 157.6 nm excitation has been investigated using the photofragment transnational spectroscopic technique. Two distinctive chemical dissociation channels have been observed: one is the binary dissociation process, OClO + hv yields ClO + O; the other one is the triple dissociation process, OClO + hv yields Cl + O + O. The branching ratio of the binary dissociation channel to the triple dissociation channel is determined to be 0.59: 0.41. Bimodal vibrational distribution of the ClO product has been observed for the OClO yields ClO (X² (Pi) ) + O(³P, ¹D) channel, implying that two distinctive dissociation routes possibly exist in the binary dissociation process. The bimodal distribution is likely caused by the two dissociation pathways from two excited electronic states: the D(²A₁) and E(²B₁) states of OClO. These arguments are further supported by the results of the anisotropy parameter measurements for the binary dissociation channels. Experimental results also show that the OClO + hv yields ClO(X² (Pi) ) + O(¹S) and OClO + hv yields ClO(A ²(Pi) ) + O(³P) channels might also exist in addition to the ClO(X²(Pi) ) + O(³P, ¹D) channel. In the triple dissociation process, experimental results show that the main product channel is the OClO + hv yields Cl(²P) + O(¹D) + O(³P) channel, while the OClO + hv yields Cl(²P) + O(³P) + O(³P) channel is the minor one. The branching ratio of these two channels is determined to be 0.89:0.11. From the modeling of the time of flight spectra of the O atom product, it is believed that the triple dissociation process of OClO is a simultaneous process within the time scale of one rotation period.

Rovibrational excitation combined with promotion of C₂HD/C₂H₂ molecules to the excited electronic trans-bent states A ¹A_u/B ¹B_u and photofragment ionization are used to generate action spectra and Doppler profiles. Rovibrational states of C₂HD and C₂H₂ are photodissociated by approximately 243.1 nm photons that also probe the H/D fragments. The yield of both H/D photofragments is greatly enhanced upon rovibrational excitation of both isotopomers. In the photolysis of C₂HD several couples of rotational transitions, terminating at the same upper rotational state, stand out from the rotational contour and the H/D branching ratio is rotationally state dependent. In C₂H₂ photolysis the transition intensities of the combination bands that involve high stretch and low bend excitation, (1410⁰2⁰) and (2031¹)⁰), are close to that of the fourth overtone of the C-H stretch, (2030⁰0⁰). In addition, in C₂H₂ some couples of rotational transitions stand out form the rotational contour, and the R(13) line of the (2030⁰0⁰) state shows anomalous intensity. The mechanism for the photoproducts preference in C₂HD and for intensity enhancement in C₂H₂ is discussed.

The photodissociation dynamics of the reaction H₂CO + hv yields H + HCO have been investigated just above the reaction threshold. Formaldehyde was excited into specific J, K_a, K_c rotational states of three vibrational levels in the A(¹A₂) state. Molecules in these states undergo internal conversion back to the X (¹A₁) ground state on which the radical fragments are formed. The ensuring distribution of rotational energy in the HCO fragment was measured as a function of the N, K_a, K_c and J equals N +/- S quantum numbers of the fragment, and also the initial v, J, K_a, K_c quantum numbers of the parent. In a previous publication we investigated the dynamics of this reaction at low available energy and concluded that when only the N and K_a quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are modeled well by phase space theory (PST). This is consistent with statistical dynamics on a bound, barrier less surface. Within approximately equals 10 cm^-1 of the energetic threshold, a centrifugal barrier affected the populations by inhibiting product states that require large orbital angular momentum. Resolution of K_c in the parent and product gave large deviations from the PST model, however little data were available to quantify this observation. In this work we have extended then umber of initially excited H₂CO levels to explore this 'K_c effect' further. We find that in the HCO K_c state or the lower energy state. This preference is consistent over all N for any particular initial H₂CO state but may very for different initial states. Over the seven initial states probed here, four favored K_c and the other three K_c. A correlation between this K_c preference and the initial state was observed: odd K_c formaldehyde states produce K_c preference in HCO and vice versa for initially even K_c states. A comparison with one previous observation of this effect is presented, however no concrete explanation can be offered at this stage.

The deperturbation analysis of ReN and nonadiabatic dissociation dynamics of BrCl and BrNO are reported. In the case of ReN, couplings between different electronic states changes apparent molecular properties such as rotational constants and excited state lifetimes. An experimental approach which enabled us to separated the contributions to the mixed states from specified energies and the spin-orbit branching ratio was measured as a function of the excitation energy. The correlation of the spin-orbit states of the photofragments was established. Such observations enabled us to establish nonadiabatic dissociation mechanisms. In both the spectroscopy and dissociation dynamics studies, the coupling between the nuclear and electronic degrees of freedom was investigated.

Since the advent of laser spectroscopy and molecular beam techniques, the photodissociation of small molecules has been intensely studied. Emphasis has been placed on investigating the applicability of statistical theories, in particular for barrierless unimolecular dissociation . Various statistical theories have been devised in order to explain reaction rates and product distributions of unimolecular reactions, especially those in which there is no potential energy barrier to products.In phase space theory (PST), all accessible product states have equal probability to be populated; the only restrictions are those of energy and angular momentum conservation. Recently, an extension of both classical and quantum PST was proposed in order to calculate v*j correlations in the products of a photodissociation reaction. Separate statistical ensembles (SSE) imposes further restrictions on the phase space by constraining energy flow between vibration and overall rotation. For photolysis energies that do not allow for the formation of vibrationally excited products, PST and SSE are identical. Another type of constraint on the phase space can be achieved by using the statistical adiabatic channel model which assumes that each vibrational state of the parent molecular adiabatically correlates to a specific rovibrational state of the fragments. This model has been sued with success on the photodissociation of HOOH. Ketene is a molecule whose photochemistry has been extensively studied both experimentally and theoretically. The two lowest electronic states of methylene are energetically accessible in the 308 nm photolysis of ketene.

The dissociation of ketene on its singlet potential energy surface has been studied using a two-step photoexcitation, followed by state-selective detection of the CH₂ radical via laser-induced fluorescence (LIF). For the first step near 2.8 micrometers , a commercial optical parametric oscillator was actively stabilized with an external reference etalon, allowing scan durations of up to 1 hour. By tuning the wavelength of the UV photolysis laser, photofragment excitation spectra of high energy selectivity were recorded. Comparison with phase space theory calculations shows that the projection K of the angular momentum onto the molecular axis is strongly mixed for K > 0 and that for K equals 0 the extent of mixing increases with J.

A technique is presented for studying state-specific picosecond unimolecular reaction dynamics of small molecules in their ground electronic states, and hence their thermal unimolecular reactions. Formyl fluoride (HFCO) is chosen for its unique properties. HFCO dissociates over a barrier of 17,200 +/- 1,400 cm^-1 to form HF + CO. It has been shown that intramolecular vibrational energy redistribution (IVR) is far from complete at such energies. Thus substantial state- and mode-selectivity are exhibited in vibrational couplings and dissociation rates. Stimulated emission pumping (SEP) is used to selectivity populate single rovibrational levels in the electronic ground state. A third, variably delayed laser pulse, probes the depleting population of molecules from this level by laser induced fluorescence. The high temporal resolution of the dump and the probe laser pulses is realized by two distributed- feedback dye lasers amplified in (beta) -barium borate crystals serving as optical parametric amplifiers. The complete system is pumped by one active passive mode-locked Nd:YAG laser. The system delivers two independently tunable 16 ps laser pulses with a bandwidth of 1.48 cm^-1 and energies of up to 1 mJ.

Investigations of fragmentation of uranium hexafluoride molecules in radiation field of the optically pumped CF₄ laser are carried out. The optical pumping was made by the pulse-periodic TEA CO₂ laser. The optical cell of the CF₄ laser was cooled by nitrogen vapor with automatic stabilization of the temperature. The maximum radiation energy carried by a single CF₄ laser pulse is 250 mJ and the average power at a pulse repetition frequency of 50 Hz is up to 6 W. It was found that besides the direct fragmentation of UF₆ molecules, the bimolecular V-V exchange realizes most probably at collision of highly excited molecules, consequently the vibronic energy of one of the molecules can exceed the dissociation threshold. Increase by an order of the UF₆ molecules photofragmentation efficiency at presence of hydrogen can be explained by high chemical activation because of reverse electron relaxation of highly vibronic excited UF₆ molecules. There is also ascertained that polymerization of UF₅ radicals appears to be the most high-speed of al secondary reactions. In the main part the polymers ar found on the cell surface having number of the radicals in range from 2 to approximately 10.

A comparative study of the hexamethyldisiloxane (HMDSO) molecule photofragmentation induced by laser multiphotonic (MPI) and synchrotron monophotonic (SR) processes is presented. The HMDSO sample was effusively expanded into the vacuum chamber and fragmented by either laser or synchrotron irradiation. The resulting ions were detected by a time-of- flight spectrometer using both electron-ion and ion-ion coincidence techniques. The parent ion has not been observed in both processes suggesting its instability. MPI induced fragmentation is characterized by a high ionic yield (IY) in the lighter fragments region. The MPI atomization is severe generating ions like C⁺ and Si⁺ that are absent from the SR spectra. The doubly-charged ions SiOSi(CH₃)₂⁺⁺ and SiOSi(CH₃)₄⁺⁺ are observed in the SR spectra. SR and MPI fragmentation have a common main route: the methyl group ejection yielding m/q equals 147,148,149 and m/q equals 15. The first presents a higher IY suggesting that the positive charge stays preferentially with the more massive fragment. Through MPI there is another route: the Si-O bond breakage yielding m/q equals 73,74,75 and m/q equals 89 (Si(CH₃)₃⁺ and OSi(CH₃)₃⁺. The metastable doubly charged ions were SiOSiC_1,2,3,6H_n⁺⁺ and OSiC₃H_n⁺⁺ in the SR case; and a wider fragment mass range was observed through MPI.

Point-charge distributions are used to represent the time- averaged electron-nucleus interaction in the Kramers- Henneberger frame. This allows the use of standard quantum chemistry programs to calculate the dressed atomic energies. We consider the two cases of linearly and circularly polarized fields. Applications are made to hydrogen and its negative anion.

We derive a simple and efficient formula, from a second- order perturbation theory, for simulating pump-probe signals. The resulting formula clearly separates the pump and probe steps, describing them as a simple filtering process, and the evolution of the wavepacket which only takes place on the excited electronic state, where we look for dynamical information. The physical implications of this model for standard pump-probe experiments and also for pump- probe electron spectroscopy are discussed. The formula is applied to the HgAr van der Waals complex and is compared against an exact calculation.

The alignment dynamics of HCN in intense laser fields are studied numerically, using time-dependent quantum mechanics. The alignment, with respect to the laser field polarization vector, is measured from the angular distribution of the molecule using the half-angle (theta) _1/2. It is shown that it can be achieved on a sub-picosecond timescale. The role of the vibration of the two molecular bonds is investigated, as well as the effect of the permanent and field-induced dipole moments on alignment.

Angular resolved kinetic energy distributions of fragments resulting from dissociation induced by intense, short, linearly polarized laser pulses are calculated using an accurate 3D Fourier transform method in spherical coordinates. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field polarization vector. But, unexpectedly, increasing the field strength may also produce less aligned fragments at the higher kinetic energies of the multiphoton above threshold dissociation spectrum. H₂⁺ photodissociating by interaction with an Nd:YAG laser at (lambda) equals 532 nm and for intensities of 10-50 TW/cm², is taken as an illustrative example, for which some angular resolved experimental spectra are available. A comprehensive interpretation is provided within the field dressed Floquet picture by referring to two strong field mechanisms; namely the potential barrier lowering, and the non-adiabatic transitions. Calculations are presented for three specific initial vibrational states leading to strongly anisotropic angular distributions which are discussed.

Bimolecular state-selective chemistry has been performed using the technique of crossed molecular beams. Systems of current study involve transition metal atoms (M) interacting with small hydrocarbons, in particular: Mo + CH₄, Zr + C₂H₄, and V + C₂H₄. Atomic metal reactants are prepared state-specifically by laser excitation followed by radiative decay. Nascent products were allowed to drift to a triply differentially pumped detector where they were ionized by either conventional electron impact or VUV photoionization and counted to obtain product angular and velocity distributions. C-H bond activation was observed for the former two systems, but not for the V systems. This behavior is rationalized in terms of the electronic configurations of the atomic reactants.

The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for ground and spin-orbit excited halogen atom formation have been measured as a function of excitation wavelength, as have the angular distributions for both fragmentation channels in both hydrogen halides. In the case of HI, the branching ratio measurements - which show a marked excitation wavelength dependence - reinforce previous findings at excitation wavelengths (lambda) < 250 nm, but at longer wavelengths show larger relative yields of spin- orbit excited iodine (I*) atoms than reported hitherto. The perpendicular nature of the electronic transition yielding ground state H + I products is confirmed but, again in contradiction to previous results, the angular distribution measurements show that the I* atoms arise almost entirely via a parallel photoexcitation pathway. HBr photolysis in the wavelength range 200 < (lambda) < 253 nm yields Br*/Br spin-orbit branching ratios consistently approximately 0.22; as in HI, all ground state products formed in this wavelength range derive from a perpendicular photoexcitation process, but the angular anisotropy of the fragmentation yielding excited Br* atoms shows a marked wavelength dependence. The observations are reviewed in the light of current knowledge of the repulsive excited states contributing to the near UV absorption spectra of the hydrogen halides.

An intrinsically time-resolved version of frequency- modulation (FM) spectroscopy has been recently developed and applied to the study of gas-phase photodissociation dynamics. Transient FM spectroscopy allows low background detection of radical species with shot-noise limited sensitivities, time resolution sufficient for detection of collision-less photoproducts, and frequency resolution characteristics of single-mode cw lasers. Methods for the quantitative analysis of Doppler-broadened FM line shapes to give velocity and rotational polarization information have been established permitting the measurement of scalar and vector properties of molecular fragmentation in exquisite detail. Several recent examples of the application of transient FM spectroscopy will be presented and discussed, including correlated scalar distributions in the dissociation of ketene from CH₂ Doppler profiles, and the full vector correlation analysis of CN fragments arising from ICN dissociation.

The non-resonant ionization and dissociation of D₂ by intense 532-nm laser light is studied by a variation of the 'ion imaging' technique called 'velocity mapping'. Images of the both the photoelectrons and D* photofragments are obtained and analyzed at two different laser intensities. Results are compared to previous studies and several differences are discussed.

Ionization by the electric field of intense laser radiation offers some advantages to study the dynamics of ultrafast chemical reactions in pump-probe experiments: any electronically excited state is expected to give an enhanced ion signal independent of its electronical and vibrational nature, depending mainly on its ionization potential; ionization is followed by extensive fragmentation which providers additional information on transient intermediate states and/or species. We applied this approach to study the femtosecond dynamics of a number of photoinduced isomerization and dissociation reactions in the gas phase including electrocyclic ring opening of 1,3-cyclohexadiene, pericyclic hydrogen migration of 1,3,5-cycloheptatriene, UV photodissociation of metal hexacarbonyls. Monitoring the pump-probe delay kinetics of the parent and various fragment ions, we are able to follow the molecule from state to state all along the primary photochemical reaction path which is completed within 200 fs. The common feature of these different chemical reactions is the ultrafast passage from S₁ to S₀ within a time in the range of 40 to 80 fs. Such ultrafast processes can only take place, if there is a continuous pathway between the two potential surfaces. The observations are therefore a strong support for the S₁/S₀ conical intersections predicted by quantum chemistry.

Stimulated Raman pumping has been used to produce strongly oriented, vibrationally excited acetylene molecules. The decay and transfer of the orientation to neighboring rotational states has been observed and characterized.

Theoretical and experimental aspects of brute force orientation in pyridazine and ICN are discussed. Using wave functions of symmetric tops as basis functions, Stark effect of the orientation field on energy levels, molecular axes, and electronic transitions are calculated. The obtained results are in qualitative agreement with experimental observations. Resonantly enhanced multiphoton ionization of pyridazine demonstrated enhancement of 40 percent in overall ionization yield as the polarization direction of the resonant laser was changed form parallel to perpendicular to the orientation field. At 250 and 266 nm, photodissociation of ICN is dominated by a parallel. Transition, and the yield of I* was enhanced as the laser was polarized parallel to the orientation field. These result demonstrate the feasibility of brute force orientation and have laid the foundation for potential applications of this technique in studies of complex systems.

Large electric fields are being used in our laboratory to orient molecules by brute force. Under the rotationally cold conditions of a molecular beam, the dipole interaction with a static electric field can be made large with respect to the rotational energy such that the molecules orient with this transition moment along the electric field direction. In this paper we will discuss the use of IR laser spectroscopy to probe this orientation and then go on to consider the use of these oriented beams in the study of photodissociation of hydrogen bonded complexes.

The entrance channel to the OH + H₂ yields H₂O + H hydrogen abstraction reaction has been probed with unprecedented detail through IR overtone spectroscopy of binary OH-H₂/D₂ complexes. IR-UV double resonance techniques have been used to obtain rotationally resolved vibrational overtone spectra for ortho-H₂-OH as well as ortho- and para-D₂OH. Assignment of the spectra has been aided by bound state calculations based on the ab initio potential of Clary, Werner, and coworkers. The pure overtone stretch and combination bands involving intermolecular bending vibrations at energies up to the OH(v equals 2) + H₂/D₂ dissociation limit have been observed. The intermolecular excitations can drastically alter the relative orientation of the reactants within the complex, producing some that resemble the transition state structure. Direct time-domain measurements show that vibrationally activated OH-H₂ complexes have a lifetime of 115(13) ns for the pure overtone stretch and an upper limit of 5 ns for OH-D₂. The 20 times faster decay for OH-D₂ arises form near-resonant vibrational energy transfer from OH to D₂. Finally, IR optical pumping provides sufficient population transfer to enable the rotational distribution of the OH(v equals 1) products from inelastic scattering to be determined.

Collisions between argon and nitric oxide were studied in crossed supersonic beams. Resonance enhanced multiphoton ionization was used to determine post collision relative populations of many rotational, spin-orbit, and (Lambda) doublet states. A preference for the production of the II(A'') (Lambda) doublet component in the collisions was observed.

Detailed time-resolved photodissociation and caging dynamics are reported for an I₂(OCS)₁₁ model system. The observed product channel-dependent nuclear coherence in the dissociated chromophore reflects complex dynamics of the solvent cage. The evolving pump-probe product distribution offers the possibility of incoherent control of two-photon dissociation pathways by appropriately delaying the probe laser pulse. As an example of such control, I₂(OCS₂ is produced most effectively by a limited set of pump-probe excitations. We emphasize generality of these results that relate to caging dynamics in nay cluster ions.

The photodissociation dynamics of gas phase I₃ following 390 nm excitation are studied with femtosecond photoelectron spectroscopy. Both I and I₂ photofragments are observed; the I₂ exhibits coherent oscillations with a frequency corresponding to an average vibrational level ((upsilon) ) equals 67. The oscillations dephase by 4 ps and rephase by approximately 45 ps. This can be understood by considering the dynamics of a coherent superposition of vibrational states on the anharmonic I₂ potential. Our results represent the first example of coherently produced photodissociation products undergoing dynamical rephasing. Also for the first time the gas phase frequency of ground state I₃ is determined via a resonance impulsive stimulated Raman scattering process. Finally, the photoelectron spectrum of I₃ yields the vertical detachment energy of I₃ and the spacing between the first two electronic states of the neutral I₃ molecule. The photodissociation dynamics of gas phase I₃ differ considerably from those observed in condensed phase experiments.

Velocity imaging, an improvement of the ion imaging method, is described and applied to a study of photodissociation of molecular oxygen. The electrostatic immersion lens introduced in this technique has the special property of projecting out the velocity information of a fragment formed in a photodissociation process, independent of the initial position of the fragment. This results in better image quality, thus more detailed information on the dynamics of collision and half-collision events. Photodissociation of molecular oxygen in the region of the Herzberg and Schumann- Runge continua using velocity imaging is discussed.

The hydrogen-atom product channel in the ultraviolet photodissociation of HN3, HN3( X 1A') + by —* H + N3(X 211), is investigated at a photolysis wavelength of 248 nm. The center-of-mass (CM) translational energy distribution of the H and N3( X 211) fragments is determined by the high-n Rydberg H-atom time-of-flight spectroscopy. The CM translational energy release peaks near the maximum available energy, with its average (Etrans) (J.7tEavaii. A strongly anisotropic product angular distribution was observed, with the anisotropy parameter 3 -0.8. The obtained information indicates a perpendicular electronic transition (A1A' f— X1A) of HN3 at 248 nm and a rapid dissociation via the repulsive excited-state surface. Bend and stretching vibration excitations of the photofragment N3 are observed. The upper limit of the bond energy D0(H-N3) is estimated to be 88.7 0.5 kcal mo11. Keywords: photodissociation, HN, H atom, N3.

The electronic spectra of SF₂ radical were experimentally investigated by using Resonance-Enhanced Multiphoton Ionization in the excitation wavelength range of 245-365 nm. A pulsed dc discharge technique was applied to generate the neutral SF₂ in the supersonic beam of Ar seeded with SF₆. From the recorded laser excitation resonance spectra at the mass-electron ratio corresponding to SF₂⁺, ten electronic band system of SF₂ including three series of Rydberg states and two valence states were observed, and most of them were vibrationally assigned. A fit of Rydberg formula to the ns origins suggest the adiabatic ionization potential for neutral SF₂ being 10.02 eV.