Wei-Bin Lee

Productions of I, I*, and C2H5 in the A-band photodissociation of ethyl iodide in the wavelength range from 245 to 283 nm by using ion-imaging detection.

Photodissociation dynamics of ethyl iodide in the A band has been investigated at several wavelengths between 245 and 283 nm using resonance-enhanced multiphoton ionization technique combined with velocity map ion-imaging detection. The ion images of I, I(*), and C(2)H(5) fragments are analyzed to yield corresponding speed and angular distributions. Two photodissociation channels are found: I(5p (2)P(3/2))+C(2)H(5) (hotter internal states) and I(*)(5p (2)P(1/2))+C(2)H(5) (colder). In addition, a competitive ionization dissociation channel, C(2)H(5)I(+)+h nu-->C(2)H(5)+I(+), appears at the wavelengths <266 nm. The I/I(*) branching of the dissociation channels may be obtained directly from the C(2)H(5) (+) images, yielding the quantum yield of I(*) about 0.63-0.76, comparable to the case of CH(3)I. Anisotropy parameters (beta) determined for the I(*) channel remain at 1.9+/-0.1 over the wavelength range studied, indicating that the I(*) production should originate from the (3)Q(0) state. In contrast, the beta(I) values become smaller above 266 nm, comprising two components, direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probabilities are determined to be 0.24-0.36, increasing with the wavelength. A heavier branched ethyl group does not significantly enhance the I(5p (2)P(3/2)) production from the nonadiabatic contribution, as compared to the case of CH(3)I.

Photodissociation dynamics of propionyl chloride in the ultraviolet region

Velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect primary photodissociation of propionyl chloride. In one-color experiments at 235 nm, the Cl and Cl(*) fragments are produced rapidly, leading to a fraction of translational energy release of 0.37 and 0.35, anisotropy parameters of 1.1 and 0.8, and quantum yield of 0.67 and 0.33, respectively, when initial excitation of the (1)(n, pi(*))(CO) band is coupled to the (1)(n(Cl), sigma(C-Cl)(*)) repulsive configuration. The resulting propionyl radical with sufficient internal energy may undergo secondary dissociation to yield CO that is characteristic of an isotropic distribution. The REMPI spectra of the CO (0,0) and (1,1) bands are measured, giving rise to a Boltzmann rotational temperature of 1200 and 770 K, respectively, and a Boltzmann vibrational temperature of 2800 K. A minor channel of HCl elimination is not detected, probably because of predissociation in two-photon absorption at 235 nm. In two-color experiments comprising an additional 248 nm photolyzing laser, Cl and Cl(*) are produced with a fraction of translational energy release of 0.43 and 0.40 and anisotropy parameters of 1.0 and 0.6, respectively. The secondary production of CO is not observed although the internal energy partitioned in the propionyl radical is in the proximity of the dissociation barrier. In either experimental scheme, a small component appearing in the center of the Cl and Cl(*) images is proposed to stem from ground state dissociation via internal conversion.

Nonadiabatic Transition in the A-Band Photodissociation of Ethyl Iodide from 294 to 308 nm by Using Velocity Imaging Detection

Photodissociation dynamics of ethyl iodide in the A-band is investigated at wavelengths between 294 and 308 nm using resonance-enhanced multiphoton ionization technique combined with velocity imaging detection. The I/I* branching, translational energy disposals, anisotropy parameters, and curve crossing probabilities of the dissociation channels are determined. The I(5p(2)P(3/2))-product channel is found to have hotter internal states of C(2)H(5), and the I*(5p(2)P(1/2)) channel is accompanied by colder C(2)H(5). Anisotropy parameters (beta) for the I* channel remain at 2.0, indicating that the I* production should originate from the (3)Q(0) state. The beta values are from 1.6 to 1.9 in the I-product channel, which comprises two components of direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probability rises rapidly around the conical intersection but remains almost constant after passing through the curve crossing. The (1)Q(1) and (3)Q(0) states in the exit region are thus expected to cross almost parallel along the dissociation coordinate. As compared to the case of CH(3)I, the nonadiabatic transition probabilities are slightly enhanced by an ethyl-substituted group.

Elimination mechanisms of Br-2(+) and Br+ in photodissociation of 1,1-and 1,2-dibromoethylenes using velocity imaging technique

Elimination pathways of the Br(2)(+) and Br(+) ionic fragments in photodissociation of 1,2- and 1,1-dibromoethylenes (C(2)H(2)Br(2)) at 233 nm are investigated using time-of-flight mass spectrometer equipped with velocity ion imaging. The Br(2)(+) fragments are verified not to stem from ionization of neutral Br(2), that is a dissociation channel of dibromoethylenes reported previously. Instead, they are produced from dissociative ionization of dibromoethylene isomers. That is, C(2)H(2)Br(2) is first ionized by absorbing two photons, followed by the dissociation scheme, C(2)H(2)Br(2)(+) + hv→Br(2)(+) + C(2)H(2). 1,2-C(2)H(2)Br(2) gives rise to a bright Br(2)(+) image with anisotropy parameter of -0.5 ± 0.1; the fragment may recoil at an angle of ∼66° with respect to the C=C bond axis. However, this channel is relatively slow in 1,1-C(2)H(2)Br(2) such that a weak Br(2)(+) image is acquired with anisotropy parameter equal to zero, indicative of an isotropic recoil fragment distribution. It is more complicated to understand the formation mechanisms of Br(+). Three routes are proposed for dissociation of 1,2-C(2)H(2)Br(2), including (a) ionization of Br that is eliminated from C(2)H(2)Br(2) by absorbing one photon, (b) dissociation from C(2)H(2)Br(2)(+) by absorbing two more photons, and (c) dissociation of Br(2)(+). Each pathway requires four photons to release one Br(+), in contrast to the Br(2)(+) formation that involves a three-photon process. As for 1,1-C(2)H(2)Br(2), the first two pathways are the same, but the third one is too weak to be detected.

Photodissociation of (ICN)(2) van der Waals dimer using velocity imaging technique.

Photodissociation of (ICN)(2) dimer from 265 to 270 nm are studied using time-of-flight mass spectrometry combined with velocity imaging technique. Both I(+) and I(2) (+) ions are found in the mass spectra. The I(2) (+) ions result from (1+1) resonant ionization of the neutral I(2) fragment that is produced in the photodissociation of dimer, but not from dissociative ionization of (ICN)(2); i.e., (ICN)(2) (+)+hnu-->I(2) (+)+2CN. The dissociation channels of I(2) (+) leading to I(+) are all found with parallel character. The total kinetic energy distributions and anisotropy parameters of the I(+) channels produced by (ICN)(2) are almost the same as those from a neutral I(2) sample, thereby confirming that the I(2) fragments are obtained in cold state. With the aid of ab initio calculations, a plausible dissociation mechanism is proposed.

Photodissociation of cis-, trans-, and 1,1-Dichloroethylene in the Ultraviolet Range: Characterization of Cl(2PJ) Elimination

By using photofragment velocity imaging detection coupled with a (2 + 1) resonance-enhanced multiphoton ionization technique, the elimination channel of spin-orbit chlorine atoms in photodissociation of cis-, trans-, and 1,1-dichloroethylene at two photolysis wavelengths of 214.5 and 235 nm is investigated. Translational energy and angular distributions of Cl((2)P(J)) fragmentation are acquired. The Cl((2)P(J)) fragments are produced by two competing channels. The fast dissociation component with higher translational energy is characterized by a Gaussian distribution, resulting from a curve crossing of the initially excited (pi, pi*) state to nearby repulsive (pi, sigma*) and/or (n, sigma*). In contrast, the slow component with a lower translational energy is characterized by a Boltzmann distribution, which dissociates on the vibrationally hot ground state relaxed from the (pi, pi*) state via internal conversion. cis-C(2)H(2)Cl(2) is found to have a larger branching of Boltzmann component than the other two isomers. The fraction of available energy partitioning into translation increases along the trend of cis- < trans- < 1,1-C(2)H(2)Cl(2). This trend may be fitted by a rigid radical model and interpreted by means of a torque generated during the C-Cl bond cleavage. The anisotropy parameters are determined, and the transition dipole moments are expected to be essentially along the C horizontal lineC bond axis. The results are also predicted theoretically. The relative quantum yields of Cl((2)P(J)) have a similar value for the three isomers at the two photolysis wavelengths.

Photodissociation of dibromobenzenes at 266 nm by the velocity imaging technique

A velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the primary Br((2)P(3/2)) fragment in the photodissociation of o-, m-, and p-dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o- and m-dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*) state followed by predissociation along a repulsive triplet (n,sigma*) state localized on the C-Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53-0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p-dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet-triplet crossing energy. The fractions of translational energy release are determined to be 6-8 and 29-40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.

(1+1) Resonance-Enhanced Multiphoton Ionization and Photodissociation Study of CS2 via the 1B2 State

(1+1) Resonance-enhanced multiphoton ionization (REMPI) spectra of CS(2) and molecular dissociation dynamics are investigated using a time-of-flight mass spectrometer equipped with velocity imaging detection. The REMPI spectra via a linear-bent 1Sigma(g)+-->(1)B(2)(1Sigma(u)+) transition are acquired in the wavelength range of 208-217 nm. Each ro-vibrational band profile of the (1)B(2)(1Sigma(u)+) state is deconvoluted to yield the corresponding predissociative lifetime from 0.3 to 3 ps. Upon excitation at 210.25 and 212.54 nm, the resulting images of S(+) and CS(+) fragments are analyzed to give individual translational energy distributions, which are resolved into two components corresponding to the CS+S((3)P) and CS+S((1)D) channels. The product branching ratios of S((3)P)/S((1)D) are evaluated to be 5.7+/-1.0 and 9.6+/-2.5 at 210.25 and 212.54 nm, respectively. Despite the difficulty avoiding the effect of multiphoton absorption, the molecular dissociation channel is verified to prevail over the dissociative ionization channel of CS(2). The anisotropy parameters for the triplet and singlet channels are determined to be approximately 0.8 and 1.1-1.3, respectively, suggesting that the predissociative state should have a bent configuration with a short lifetime.

Ionization and dissociation mechanisms of ketene using resonance-enhanced multiphoton ionization mass spectrometer: (2+2) versus (2+1) schemes

By using a resonance-enhanced multiphoton ionization (REMPI) technique, we have studied ionization and photodissociation mechanisms of ketene. Prior to ionization, the jet-cooled ketene is first excited at a wavelength 355 nm to a 3py,00 Rydberg state through a two-photon absorption. The (2+2) and (2+1) REMPI schemes may be distinguished depending on the impinging laser energy. If the (2+2) REMPI process dominates, the ketene ion is produced by the autoionization of a superexcited state, which lies in a rovibrationally excited Rydberg state. The autoionization then occurs due to energy transfer from nuclear to electronic degrees of freedom. The CH2+ is fragmented following two schemes. One is a consecutive process, i.e., the fragment ion is produced from the autoionized ketene. This conclusion is supported by a series of measurements of pulse field and pressure effects in this work. The factors of pulse field and collisions may enhance the autoionization rate significantly, imposing the same influence on ...