Core-level spectroscopy of the photodissociation process of BrCN molecule

Fewest-switches surfacing hopping(FSSH) simulations have been performed with the high-level multi-reference electronic structure method to explore the coupled electronic and nuclear dynamics upon photoexcitation of cyanogen bromide(BrCN). The potential energy surfaces(PES) of BrCN are charted as functions of the Jacobi coordinates(R, θ). An indepth examination of the FSSH trajectories reveals the temporal dynamics of the molecule and the population changes of the lowest twelve states during BrCN's photodissociation process, which presents a rich tapestry of dynamical information.Furthermore, the carbon K-edge x-ray absorption spectroscopy(XAS) is calculated with multi-reference inner-shell spectral simulations. The rotation of the CN fragment and the elongation of the C–Br bond are found to be the reason for the peak shifting in the XAS. Our findings offer a nuanced interpretation for inner-shell probe investigations of BrCN, setting the stage for a deeper understanding of the photodissociation process of cyanogen halides molecules.

Imaging Isocyanic Acid Photodissociation at 193 nm:the NH(a^1△)+CO(X^1∑^+)Channel

The photodissociation dynamics of isocyanic acid （HNCO） has been studied by the time- sliced velocity map ion imaging technique at 193 nm. The NH（a1△） products were measured via （2＋1） resonance enhanced multiphoton ionization. Images have been accumulated for the NH（a1△） rotational states in the ground and vibrational excited state （v=0 and 1）. The center-of-mass translational energy distribution derived from the NH（a1△） images implies that the CO vibrational distributions are inverted for most of the measured 1NH（v｜j） internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N-C bond cleavage. A bimodal rotational state distribution of CO（v） has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways.

Effect of Parent Internal Excitation on Product State Distribution in Methyl Radical Photodissociation at 212.5nm

Photodissociation dynamics of the CH3 radical at 212.5 nm excitation has been studied experimentally using the H atom Rydberg tagging time-of-flight method. CH3 radicals are produded by photodissociation of CH3I at 266 nm. Translational energy distribution and angular distribution for the CH2 product from CH3 photodissociation at different vibrational levels via the 3s Rydberg state have been measured. From these distributions, product J state distributions are obtained for photodissociation of different vibrationally excited CH3 radicals. The effect of parent vibrational as well as rotational excitation on the dissociation dynamics of CH3 is also investigated in detail. Experimental results in this work show that parent vibrational excitation in the umbrella mode has a significant effect on both rotational excitation and angular distribution of the CH2 product, while parent rotational excitation has obvious effect only on the angular distribution of CH2 product.

Photodissociation Dynamics of 2-1odotoluene Investigated by Femtosecond Time-Resolved Mass Spectrometry

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 3804-50 fs was measured from the rising time of the co-fragments of toluene radical （C7H7） and iodine atom （I）, which is attributed to the averaged time needed for the C-I bond breaking for the simultaneously excited nσ and ππ＊ states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated hσ＊ and ππ＊ states. And a rise time of 4004-50 fs is extracted from the fitting of time-dependent I＋ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.

Photodissociation Spectra of OCS＋ via B2∑＋←X2П Transitions

In the wavelength range of 231-275 nm, we have studied the mass-resolved dissociation spectra of OCS＋ via B2∑＋←X2П3/2（000） and B2∑＋←X2П1/2（000, 001） transitions by preparing OCS＋ ions in the well-defined spin-orbit states. The spectroscopic constants of v1 （CS stretch）=828.9 （810.4） cm-1, u2 （bend）=491.3 cm-1 and v3（CO stretch）=1887.2 cm-1 for OCS＋（B2∑＋） are deduced. The observed dependence of the v2（bend） mode excitation of B2∑＋ on the spin-orbit splitting of X2П（Ω=1/2, 3/2） in the B2∑＋←-X2П transition can be attributed to the K coupling between the （000）2П1/2 and （010）2∑＋/2 vibronic levels of X2П state, which makes the B2∑＋（010）←X2П1/2（000） transition possible.

Photodissociation Study of Ca^＋-Dipropylamine Complex

Gas-phase Ca^＋-dipropylamine complex has been subjected to photodissociation （400-690 nm）. One reactive product, Ca^＋-NHC3H7, is detected only in the region of 450-528 nm, besides the evaporation fragment of Ca^＋, which is dominant throughout the whole spectral region we have studied. The photoreaction channel may be explained in terms of a metal insertion mechanism. The calculated results are in good agreement with the experimental observations.

Photodissociation and Density Functional Calculations of Small VmOn＋ Clusters

Oxygen-poor vanadium oxide clusters, V2On＋ （n=l, 2）, V3On＋ （n=l, 2, 3）, and V4O3＋, were produced by laser vaporization and were mass-selected and photodissociated with 532 and 266 nm photons. The geometric structures and possible dissociation channels of these clusters were determined based on the comparison of density functional calculations and pho- todissociation experiments. The experiments show that the dissociation of V2O＋, V2O2＋, and V3O3＋ mainly occurs by loss of VO, while the dissociation of V3O＋ and V4O3＋ mainly occurs by loss of V atom. For the dissociation of V3O2＋, the VO loss channel is slightly dominant compared to the V loss channel. The combination of experimental results and theoretical calculations suggests that the V loss channels of V3O＋ and V4O3＋ are single photon processes at both 532 and 266 nm. The VO loss channels of V2O2＋ and V3O3＋ are multiple-photon processes at both 532 and 266 nm.

Time-sliced Velocity Map Imaging Study on Photodissociation of Neopentyl Bromide and Tert-pentyl Bromide at 234 nm

We present a first velocity map imaging study on the 234 nm photodissociation dynamics of two carbon-chain branched alkyl bromides, neopentyl bromide （denoted as NPB） and tert- pentyl bromide （denoted as TPB）. Unlike the 234 nm photodissociation of the unbranched n-C5H11Br molecule where only a direct fission of the C-Br bond is involved, the branched NPB and TPB molecules exhibit one and two more independent dissociation pathways with much energy being decayed via an extensive excitation of the bending modes of the parent molecules prior to the C-Br bond fission. This observation strongly suggests that the dissociation coordinate for the two carbon-chain branched molecules is no longer solely ascribed to the C-Br stretching mode but rather a combination of the bending-stretching modes.

Photodissociation of HOD via the C1B1 State： OD/OH Branching Ratio and OD Bond Dissociation Energy

Photodissociation of jet-cooled HOD via the C state around 124 nm has been studied using the H（D）-atom Rydberg tagging time-of-flight technique. Rotational state resolved action spectrum and the product translational energy distribution spectra have been recorded for both D＋OH and H＋OD dissociation channels. Product channel OH/OD branching ratios for the individual C-X rotational transition have been determined. A comparison is also given with the B-X and A-X transitions. In addition, the dissociation energy of the OD bond in HOD has been determined accurately to be 41751.3±5 cm-1.

Photodissociation of 2-Bromobutane by Ion-velocity Map Imaging Technique

The photodissociation dynamics of 2-bromobutane has been investigated at 233.62 and 233.95 nm by ion-velocity map imaging technique coupled with resonance-enhanced multiphoton ionization. The speed and angular distribution of Br and Br＊ fragments were determined from the map images. The two Gaussian components, shown in the speed dis- tributions of Br and Br＊ atoms, are suggested to attribute to the two independent reaction paths of photodissociation for 2-bromobutane at 233.62 and 233.95 nm. The high-energy component is related to the prompt dissociation along the C-Br stretching mode, and the low-energy component to the dissociation from the repulsive mode with bending and C-Br stretching combination. The contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products （Br and Br＊） were discussed. Relative quantum yield of 0.924 for Br（2P3/2） at about 234 nm in the photodissociation of 2-bromobutane is derived.

Photodissociation of 2-Bromobutane at -265 nm by Ion-velocity Map Imaging Technique

The photodissociation dynamics of 2-bromobutane has been investigated at 264.77 and 264.86 nm by ion-velocity map imaging technique coupled with resonance-enhanced multi- photon ionization. The speed and angular distributions have been derived from the velocity map images of Br and Br^＊. The speed distributions of Br and Br^＊ atoms in the photodis- sociation of 2-bromobutane at -265 nm can be fitted using only one Gaussian function indicating that bromine fragments were produced via direct dissociation of C-Br bond. The contributions of the excited 3^Q0, 3Q1, and 1^Q1 states to the products （Br and Br^＊） were discussed. It is found that the nonadiabatic 1^Q1←3^Q0 transition plays an important role for Br photofragment in the dissociation of 2-C4HgBr at -265 nm. Relative quantum yield of 0.621 for Br（2P3/2） at -265 nm in the photodissociation of 2-bromobutane is derived. By comparing the photodissociation of 2-C4H9Br at -265 nm and that that at-234 nm, the anisotropy parameter β（Br） and β（Br^＊）, and relative quantum yield Ф（Br） decrease with increasing wavelength, the probability of curve crossing between 3 ^Q0 and 1^Q1 decreases with increasing laser wavelength.

Photodissociation Dynamics of Carbon Dioxide Cation via the Vibrationally Mediated A^2Hu,1/2 State： A Time-Sliced Velocity-Mapped Ion Imaging Study

We report on the photodissociation dynamics of CO2^＋ via its A2Пu,1/2 state using the scheme of [1＋1] photon excitation that is intermediated by the mode-selected A2Hu,1/2（Vl,V2,0） vibronic states. Photodissociation fragment exciation spectrum and images of photofragment CO＋ have been measured to obtain reaction dynamics parameters such as the available energy and the average translational energy. Combining with the potential energy functions of CO2^＋, the dissociation mechanism of CO2^＋ is discussed. The conformational variation of CO2^＋ from linear to bent on the photodissociation dynamics of CO2^＋ is verified.

Control of the photoionization/photodissociation processes of cyclopentanone with trains of femtosecond laser pulses

A train of three equally spaced femtosecond laser pulses is employed to control the photoionization/photodissociation processes of cyclopentanone. With the increase of pulse separation, a strong modulation of product ion yield is observed. More than ten-fold changes of ion yield ratio between different products can be realized. The experimental observations further explain the compositions and formation pathways of peaks in the mass spectra. The controlling mechanisms are also discussed.

Modification of Reflectron Time-of-Flight Mass Spectrometer for Photodissociation of Mass-Selected Cluster lons

We introduce a modification of reflectron time-of-flight mass spectrometer for laser photodissociation of mass-selected ions. In our apparatus, the ions of interests were selected by a mass gate near the first space focus point and decelerated right after the mass gate, were then crossed by a laser beam for dissociation. The daughter ions and surviving parent ions were re-accelerated and analyzed by the reflectron time-of-flight mass spectrometer. Compared to the designs reported by other research groups, our selection-deceleration-dissociation-reacceleration approach has better daughter-parent-ions-separation, easier laser timing, and better overlapping between the ion beam and laser beam. We also conducted detailed cal- culations on the parent ion and daughter ion flight times, and provided a simplified formula for the calibration of daughter ion mass.

Photodissociation Dynamics of OCS at 217 nm

The S（1D2）＋CO（X1Σ＋） product channel from photodissociation of OCS at 217 nm has been measured using the DC slice velocity map imaging （VMI） technique in combination with resonance enhanced multiphoton ionization （REMPI）. Two diflerent REMPI intermediate states （1F3 and 1P1） and several pump-probe laser polarization geometries are used to detect the angular momentum polarization of the photofragmented S（1D2）. The molecular- frame polarization parameters, as well as the laboratory-frame anisotropy parameters, for individual rotational states of co-fragment CO, are determined using two diflerent full quantum theories. The measured total kinetic energy release spectrum from photodissociation of OCS indicates two dissociation channels, corresponding to the fast and slow recoiling velocities of S（1D2）, respectively. The slow channel is concluded to originate from an initial photoexcitation to the A（1A＇） state, followed by a non-adiabatic transition to the ground state. The fast channel is found to follow a coherent excitation to A（1A＇） and B（1A＇） states, where contributions of the two states are almost equal at 217 nm. The determined alignment and anisotropy parameters further indicate that the slow channel follows an incoherent excitation, while the fast channel follows a coherent excitation to A（1A＇） and B（1A＇） states with a phase di erence of π/2.

Investigation on the Photodissociation of Oxygen from Oxymyoglobin by Fluorescence Spectroscopy

Photodissociation of oxygen from oxymyoglobin（oxyMb） was investigated by means of fluorescence spectroscopy. One of the most important findings of the photodissociation of oxyMb was the discovery of two processes which were affected by excitation intensity, temperature, solvent viscosity, and excitation wavelength. Process I（PI） corresponded to oxygen escaping from the binding site at ferrous heme iron atom within the porphyrin ring into the heme pocket, whereas process II（PII） was ascribed to oxygen escaping from the heme pocket into the solvent. To elucidate this interesting phenomenon, we proposed a model that oxygen encountered two barriers on its way from the binding site at the ferrous heme iron to the solvent. Reversibility and wavelength sensitivity of the photodissociation were also observed.

Infrared Photodissociation Spectroscopy of Ti＋（CO2）2Ar and Ti＋（CO2）n （n=3-7） Complexes

Ti＋（CO2）2Ar and Ti＋（CO2）n （n=3-7） complexes are produced by laser vaporization in a pulsed supersonic expansion. The ion complexes of interest are each mass-selected in a time- of-flight spectrometer, and studied with infrared photodissociation spectroscopy. For each complex, a sharp band in the CO stretching frequency region is observed, which confirms the formation of the OTi＋CO（CO2）~_l oxide-carbonyl species. Small OTi＋CO（CO2）~_1 complexes （n_〈5） exhibit CO stretching and antisymmetric CO2 stretching vibrational bands that are blue-shifted from those of free CO and CO2. The experimental observations indicate that the coordination number of CO and CO2 molecules around TiO＋ is five. Evidence is also observed for the presence of another electrostatic bonding Ti＋（CO2）2 structural isomer for the Ti＋（CO2）2Ar complex, which is characterized to have a bent OCO-Ti＋-OCO structure stabilized by argon coordination.

A Velocity Map Ion-imaging Study on Ketene Photodissociation at 218 nm

Photodissociation dynamics of ketene at 218 nm has been investigated using the velocity map ion-imaging method. Both angular and translational energy distributions for the CO products at different rotational and vibrational states have been obtained. The 2＋1 REMPI spectrum of CO products is also obtained. The results are as bellow： （i） CO products in the first two vibrational states （ v＂=0 and v＂=1 ） exhibit significant rotational excitation. Furthermore the rotational excitation of CO at the v＂=0 level is noticeably higher than that at the v＂=1 level. （ii） It was found that the major photodissociation pathway of ketene at 218 nm is the CH2（ǎ^1A1）＋CO（X^1∑^＋） channel, while the CH2（b^1B1）＋CO（X^1∑^＋） channel and the CH2（X^3B1）＋CO（X^1E^＋） channel are also likely present, （iii） The anisotropy parameters β of CO different rovibronic states all appear to be larger than zero. No significant difference is observed at the two vibrational states,

Adiabatic Potential Energy Surfaces and Photodissociation Mechanisms for Highly Excited States of H_(2)O

Full-dimensional adiabatic potential energy surfaces of the electronic ground state X and nine excited states A,I,B,C,D,D',D'',E' and F of H_(2)O molecule are developed at the level of internally contracted multireference configuration interaction with the Davidson correction.The potential energy surfaces are fitted by using Gaussian process regression combining permutation invariant polynomials.With a large selected active space and extra diffuse basis set to describe these Rydberg states,the calculated vertical excited energies and equilibrium geometries are in good agreement with the previous theoretical and experimental values.Compared with the well-investigated photodissociation of the first three low-lying states,both theoretical and experimental studies on higher states are still limited.In this work,we focus on all the three channels of the highly excited state,which are directly involved in the vacuum ultraviolet photodissociation of water.In particular,some conical intersections of D-E',E'-F,A-I and I-C states are clearly illustrated for the first time based on the newly developed potential energy surfaces(PESs).The nonadiabatic dissociation pathways for these excited states are discussed in detail,which may shed light on the photodissociation mechanisms for these highly excited states.

A Quantum Wavepacket Study of State-to-State Photodissociation Dynamics of HOBr/DOBr

Photodissociation of HOBr is an important step in the reaction network of the depletion of ozone in stratosphere.Here,we report the first three-dimensional potential energy surfaces for the lowest three singlet states for HOBr,based on high level multi reference configuration interaction calculations.Quantum dynamics calculations are performed with a real wavepacket method,yielding not only absorption spectra but also internal state and angular distributions of the photodissociation fragments.Our results agree quantitatively with the measured total absorption cross sections of HOBr in the ultraviolet region and reproduce well the observed vibrationally cold and rotationally hot OH/OD fragments via photodissociation of HOBr/DOBr at 266 nm.In addition,we predict that the recoil anisotropy parameters for OH/OD are close to the limiting value of a parallel transition,suggesting a rapid dissociation process at 266 nm following an in-plane transition from the ground state(1^1A')to the 21A'state.This is consistent with the experimental conclusion derived from the measured rotational alignment.However,spin and electronic angular momenta need to be taken into account in the future to achieve a more quantitative agreement with experiment.Our work is expected to motivate further experimental investigations for this benchmark system.