Spin Orbit Excited Cl Research Articles

Crossed beam experiment on the validity of Born-Oppenheimer approximation in Cl(2P)+D2→DCl+D reaction

The reaction of chlorine atom Cl(2P) (Cl(2P3/2) and Cl∗(2P1/2)) with D2 was investigated at collision energy from 4.5 kcal/mol to 6.5 kcal/mol with a high-resolution crossed molecular beam apparatus using the technique of D-atom Rydberg tagging detection. The contribution from the spin-orbit excited reaction Cl∗(2P1/2)+D2, which is prohibited by Born-Oppenheimer (BO) approximation, was observed. Collision-energy dependence of differential cross sections (DCSs) near the backward scattering direction was measured. The BO-forbidden reaction Cl∗+D2 was found to be dominant at lower collision energy. As collision energy increases, reactivity of BO-allowed reaction Cl+D2 increases much faster than that of BO-forbidden reaction and becomes dominant at higher collision energy. Our experiment indicates that the additional energy of spin-orbit excitation in Cl∗ facilitates BO-forbidden reaction to pass through the barrier at lower collision energy, while BO approximation is still valid at collision energy near and above the reaction barrier. This tendency of reactivity of Cl/Cl∗+D2 is similar to the isotopic reaction of Cl/Cl∗+H2.

Mixed transitions in the UV photodissociation of propargyl chloride revealed by slice imaging and multireference ab initio calculations.

Resonance-enhanced multiphoton ionization (REMPI) and DC slice imaging were used to detect photoproducts Cl (2P3/2), spin-orbit excited Cl* (2P1/2), and C3H3 in the photodissociation of propargyl chloride at 212 and 236 nm. Cl and Cl* translational energy distributions peak at high recoil energy, suggesting impulsive energy release in the C-Cl coordinate. Near 236 nm, photofragment angular distributions show rapidly changing anisotropy across the main peak for both Cl and Cl*, indicating excitations arising from in-plane or out-of-plane π systems. At 212 nm, the distribution is broader and isotropic for both Cl products. Ionization of C3H3 at 212 nm via 1 + 1 REMPI provides additional insight into these processes. A portion of the C3H3+ distribution is momentum-matched to the Cl, but in addition, there is a component assigned to cyclic C3H3+ formed from dissociative ionization of propargyl chloride. Multireference ab initio calculations show that excitations to three triplet states contribute at the long wavelength region, while predissociation from the first excited singlet state plays a role at the shorter wavelength.

Near-threshold photodissociation dynamics of CHCl3

Energy- and angle-resolved photofragment distributions for ground-state Cl ((2)P3/2) and spin-orbit excited Cl* ((2)P1/2) have been recorded using the velocity map imaging technique after photodissociation of chloroform at wavelengths of 193 and ∼235 nm. Translational energy distributions are rather broad and peak between 0.6 and 1.0 eV. The spin-orbit branching ratios [Cl*]/[Cl] are 1 and 0.3 at 193 and 235 nm, respectively, indicating the involvement of two or more excited state surfaces. Considering the anisotropy parameters and branching ratios collectively, we conclude that the reaction at 193 nm takes place predominantly on the (1)Q1 surface, while the (3)Q1 surface gains importance at lower dissociation energies around 235 nm.

Laser-induced fluorescence study of spin–orbit quenching of Cl(2P1/2) by H2, D2, and HD gases

Employing ultraviolet laser flash photolysis in combination with pulsed vacuum ultraviolet laser induced-fluorescence spectroscopy room-temperature rate constants kQ,H2=(5.8±0.6)×10−11cm3s−1, kQ,D2=(5.6±0.6)×10−12cm3s−1, and kQ,HD=(2.7±0.3)×10−11cm3s−1 for electronically non-adiabatic quenching of spin–orbit excited Cl(2P1/2) atoms in collisions with H2, D2, and HD gases were determined, which allow for comparison with results of recent quantum-scattering calculations [14,15].

Translational energy dependence of the Cl + CH4(v b = 0, 1) reactions: a joint crossed-beam and quasiclassical trajectory study

The Cl + CH4(v b = 0, 1) reactions were studied over a wide range of collision energies, from threshold up to 20 kcal mol−1, using joint experimental and theoretical methods. Experiments were performed under crossed-beam conditions using a time-sliced velocity imaging detection method. Both the pair-correlated integral and differential cross sections were measured. Theoretically, quasiclassical trajectory calculations were performed on a highly accurate ab initio potential energy surface. The computed results show very good agreement with experimental findings. The underlying reaction mechanisms are as below: the formation of HCl(v′ = 0) products is mainly governed by direct scatterings, whereas the HCl(v′ = 1) channel is mediated by a time-delayed mechanism, likely invoking reactive resonances. In addition, the spin-orbit excited Cl*(2 P 1/2) reactivity was experimentally characterized. The results compare favourably with a recently reported reduced dimensionality quantum dynamics calculation, and with the previous reports on the isotopically analogous reactions. Possible involvement of resonances in this spin-orbit nonadiabatic process is suggested and awaits further investigations.

The photodissociation dynamics of tetrachloroethylene

We present a direct current slice imaging study of tetrachloroethylene (C(2)Cl(4)) photodissociation, probing the resulting ground state Cl ((2)P(3/2)) and spin-orbit excited state Cl* ((2)P(1/2)) products. We report photofragment images, total translational energy distributions and the product branching ratio of Cl*/Cl following dissociation at 235 and 202 nm, obtained using a two-color reduced-Doppler dissociation/probe. Near 235 nm, the Cl translational energy distribution shows a peak at the limit of the available energy, indicating a direct dissociation through a σ*(C-Cl) ← π (C=C) transition, which is superimposed on a broader underlying distribution. The ground state Cl image and associated translational energy distribution at 202 nm is broad and peaked at lower energy, suggesting either internal conversion to the ground state or a lower excited state prior to dissociation. The Cl* images are similarly broad at both wavelengths. The branching ratio is presented as a function of recoil energy, but after integration shows a near-statistical average of Cl:Cl* as 70:30 at both wavelengths. All the images are largely isotropic, with anisotropy parameters (β) of 0.05 ± 0.03.

Is spin-orbit state branching in the photodissociation of CCl 4 isotope specific?

We have reinvestigated the ultraviolet photodissociation of CCl 4 around 235 nm. The yield of spin-orbit state excited Cl*( 2P 1/2) atoms was found to be 0.26 ± 0.03, independently of the isotopic mass of the chlorine atom. No evidence of isotope specific Cl* yields was observed. Our results suggest that the previously reported isotope specificity in the photodissociation of CCl 4 does not exist.

Photodissociation of state selected BrCl+ cations: branching ratios and angular anisotropies of the Br+ product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370–420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3 P J ) + Cl(2 P J ) product channels. Ground state Br+(3 P 2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin–orbit excited Cl(2 P 1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π ← X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin–orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin–orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

Photodissociation of state selected BrCl cations: Branching ratios and angular anisotropies of the Br product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370-420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3PJ) + Cl(2PJ) product channels. Ground state Br+(3P2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(2P1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π←X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin-orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin-orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

Kinetics of the Reactions of Cl*(2P1/2) and Cl(2P3/2) Atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.

High resolution ion imaging study of BrCl photolysis in the wavelength range 330–570 nm

The photodissociation of jet-cooled BrCl molecules has been investigated at many different excitation wavelengths in the range 330–570 nm by velocity map imaging of the ground state Br and ground and spin–orbit excited Cl atom products. Image analysis confirms literature values for the energies of the ground, Br(2P3/2)+Cl(2P3/2), and first excited [Br(2P3/2)+Cl(2P1/2)] dissociation asymptotes, and provides measures of the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of parent vibrational level (v″⩽2) and excitation wavelength. The availability of such experimental data allows mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorption as a function of excitation energy, and thus deconvolution of the room temperature parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+), and C 1Π(1) excited states of BrCl. This analysis of the continuous absorption spectrum of BrCl, when considered in conjuction with existing spectroscopic data for the ground electronic state and for the bound levels supported by the B state potential, allows determination of key regions of the potential energy curves for, and transition moments to, each of these three excited states. Further wave packet calculations, which reproduce the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies very well, serve to validate the excited state potential energy functions so derived and allow estimation of the strength (∼80 cm−1) of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry.

Photodissociation dynamics of vibrationally excited CH 2Cl 2 molecules

Photodissociation of dichloromethane excited to the second C–H stretch overtone was performed by ∼235 nm photons that also tagged the Cl 2P 3/2 [Cl] and spin–orbit excited Cl 2P 1/2 [Cl *] photofragments via resonantly enhanced multiphoton ionization. The measured time-of-flight profiles together with the determined Cl */Cl branching ratio suggest fast dissociation and involvement of several upper states of different symmetries mixing via curve crossing to release both Cl and Cl *. The determined ratio is higher than that obtained previously in the direct, almost isoenergetic, photodissociation of CH 2Cl 2 implying higher non-adiabaticity for vibrationally excited than for vibrationless ground state molecules.

Absolute chlorine and hydrogen atom quantum yield measurements in the 193.3 nm photodissociation of CH3CFCl2 (HCFC-141b)

The dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH3CFCl2 (HCFC-141b), were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence (LIF) “pump-and-probe” technique. Nascent ground-state Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) as well as H(2S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl(2P3/2), Cl*(2P1/2), and H atom concentrations, values for the chlorine-atom spin–orbit branching ratio [Cl*]/[Cl]=0.36±0.08, the total chlorine atom quantum yield (ΦCl+Cl*=1.01±0.14), and the H atom quantum yield (ΦH=0.04±0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be ET(Cl)=157±12 kJ/mol and ET(Cl*)=165±12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH3CFCl+Cl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH3CFCl2→CH3CFCl+Cl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C–Cl bond cleavage is the most important primary fragmentation mechanism for CH3CFCl2 after photoexcitation in the first absorption band.

The ultraviolet photodissociation of 2-chlorobutane: an imaging study comparing state-resolved and universal probe techniques

We present an imaging study of the Cl elimination channel in the 193 nm photodissociation of 2-chlorobutane. We compare the results obtained using resonant 2+1 photoionization of the Cl( 2P 3/2) and Cl *( 2P 1/2) products with those obtained using single photon ionization of the 2-butyl radical cofragment, and find reasonable agreement between the two techniques. The product translational energy distribution is sharply peaked at ∼50% of the available energy, while the branching fraction to the spin–orbit excited Cl ∗ is 0.50. We also present results for H elimination in the 157 nm photodissociation of 2-chlorobutane. The results are interpreted using a simple impulsive model of the dissociation dynamics.

Photodissociation dynamics of phosgene: New observations by applying a three-dimensional imaging technique

The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) fragments by applying a novel technique where the three-dimensional momentum vector of a single reaction product is directly determined. The photodissociation at 235 nm produces exclusively three fragments: COCl2+hν→CO+2Cl. The kinetic energy distributions of Cl and Cl* are bimodal and exhibit a different behavior for the different spin–orbit states. Our attention was turned to the dependence of the anisotropy parameter β on the fragment velocity which was observed for the first time. For both spin–orbit states the anisotropy parameter differs clearly for slow and fast chlorine atoms, where a pronounced change from the value ∼0.7 to zero at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the theoretically limiting value and are distributed between ground and excited state Cl. These observations can be explained by a sequential decay where the first Cl fragment is released in a fast process characterized by the nonvanishing positive β parameter and a lifetime of ⩽210 fs, whereas the second Cl fragment is released after a period which is long on a rotational time scale. A significant contribution of a symmetric mechanism can be excluded.

Evidence for the onset of three-body decay in photodissociation of vibrationally excited CHFCl2

Excitation of C–H stretch overtones of CHFCl2 followed by ∼235 nm photodissociation was applied to investigate the effect of internal parent excitation on the dynamics of two- and three-body photofragmentation. The ∼235 nm photons also tagged ground Cl 2P3/2 [Cl] and spin–orbit excited Cl 2P1/2 [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer, and monitored their time-of-arrival profiles. These profiles revealed the product velocities and angular distributions of Cl35 and Cl*35 and suggest the contribution of three-body decay in photodissociation of CHFCl2 pre-excited with five quanta of C–H stretch. This is the first evidence for three-body decay in photodissociation of vibrationally excited molecules.

Isotope effects in the deactivation of O( [formula omitted]D) atoms by XCl and XF (X=H,D)

The method of time-resolved laser magnetic resonance (LMR) was used to study the deactivation of O( 1 D) by HCl, DCl, HF, and DF at room temperature. For O( 1 D)+XF(X=H, D), the effect of deuteration on the rates of physical quenching and chemical reaction was negligible. For O( 1 D)+XCl, no essential isotope effect on the deactivation rate, on the Cl+OX reaction channel probability, and on the relative quantum yields of spin–orbit excited Cl( 2 P 1/2) atoms was found. While the formation of H+ClO is found to account for 0.18±0.04 of the deactivation rate, in the deuterated case this channel was negligible (<0.03).

Exploring the spin–orbit reactivity in the simplest chlorine atom reaction

The reaction of Cl(2P)+H2→HCl+H serves as a benchmark for understanding the abstraction mechanism and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the reactivity of the low-lying spin–orbit excited Cl*(2P1/2) is unknown and has always been assumed to be negligibly small. By exploiting two different sources for generating the Cl(2P) beam, it is now found that the excited Cl*(2P1/2) atom is surprisingly more reactive to H2 than the ground state Cl(2P3/2) reagent, overturning conventional wisdom. By using a technique called Doppler-selected time of flight to directly map out the doubly differential cross sections (angle and speed), the detailed dynamical attributes for both spin–orbit states are also elucidated and contrasted for the first time for any bimolecular reaction.

The ultraviolet photodissociation of DCl: H/D isotope effects on the Cl(2P) atom spin–orbit branching

The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV absorption band (A 1 Π 1 ←X 1 Σ + ) has been investigated at five wavelengths in the range 200–220 nm. Ground state Cl( 2 P 3/2 ) and spin–orbit excited Cl ∗ ( 2 P 1/2 ) photofragments were monitored using (2+1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant H/D isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.

Chlorine atom formation dynamics in the dissociation of CH3CF2Cl(HCFC-142b) after UV laser photoexcitation

The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2Cl(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique at room temperature. The parent molecule was excited at the ArF excimer laser wavelength (193.3 nm) and nascent ground state Cl(2P3/2) and spin-orbit excited Cl*(2P1/2) photofragments were detected under collision-free conditions via laser induced fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable over the wavelength range 133.5–136.4 nm, was generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a source of well-defined Cl(2P3/2) and Cl*(2P1/2) concentrations, values for the total Cl atom quantum yield (ΦCl+Cl*=0.90±0.17) and the [Cl*]/[Cl] branching ratio 0.39±0.11 were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the average relative translational energy of the fragments could be determined to be 125±25 kJ/mol. The corresponding value fT=0.48±0.10 of the fraction of total available energy channeled into product translational energy was found to be (within experimental uncertainty) in agreement with the result fT=0.39 of a dynamical simulation assuming a repulsive model for single C–Cl bond cleavage. Both the measured total Cl atom quantum yield and the energy disposal indicates that direct C–Cl bond cleavage is a primary fragmentation mechanism for CH3CF2Cl after photoexcitation at 193.3 nm.