Heavy hydrides: H2Te ultraviolet photochemistry

The room-temperature ultraviolet absorption spectrum of H2Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high-n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH(2Pi1/2) selectively relative to the TeH(2Pi3/2) ground state. This is attributed to the role of the 3A' state, which has a shallow well at large R(H-TeH) and correlates to H+TeH(2Pi1/2). Note that the 2Pi1/2 state is analogous to the 2P1/2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3A' state is crossed at large R only by 2A", with which it does not interact. The character of 3A' at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, 2Pi1/2 has a higher degree of spherical symmetry than does 2Pi3/2 (recall that I(2P1/2) is spherically symmetric), and consequently 2Pi1/2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3A'<--X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3A'. The main absorption feature that is peaked at approximately 240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4A" surface. The 4A"<--X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0. For H2Te and TeH, these are 65.0+/-0.1 and 63.8+/-0.4 kcalmol, respectively, in good agreement with predictions that use high-level relativistic theory.     

Photodissociation of the BrO radical using velocity map ion imaging: excited state dynamics and accurate D0(0)(BrO) evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-281.5 nm) and below (355 nm) the A (2)Pi(3/2) state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O((1)D(2)) threshold the contribution of the direct excitation to states other than the A (2)Pi(3/2) state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O((1)D(2)) threshold provides an accurate and direct determination of the A (2)Pi(3/2) state dissociation threshold of 35418+/-35 cm(-1), leading to a ground state bond energy of D(0)(0)(BrO)=55.9+/-0.1 kcal/mol.     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.     

Photoionization and photodissociation dynamics of the B 1sigmau + and C 1Piu states of H2 and D2

The photoionization and photodissociation dynamics of H(2) and D(2) in selected rovibrational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states have been investigated by velocity map ion imaging. The selected rotational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states are prepared by three-photon excitation from the ground state. The absorption of fourth photon results in photoionization to produce H(2)(+) X (2)Sigma(g)(+) or photodissociation to produce a ground-state H(1s) atom and an excited H atom with n >or= 2. The H(2) (+) ion can be photodissociated by absorption of a fifth photon. The resulting H(+) or D(+) ion images provide information on the vibrational state dependence of the photodissociation angular distribution of the molecular ion. The excited H(n >or= 2) atoms produced by the neutral dissociation process can also be ionized by the absorption of a fifth photon. The resulting ion images provide insight into the excited state branching ratios and angular distributions of the neutral photodissociation process. While the experimental ion images contain information on both the ionic and neutral processes, these can be separated based on constraints imposed on the fragment translational energies. The angular distribution of the rings in the ion images indicates that the neutral dissociation of molecular hydrogen and its isotopes is quite complex, and involves coupling to both doubly excited electronic states and the dissociation continua of singly excited Rydberg states.     

Photoionization and photodissociation of HCl(B 1Sigma+, J=0) near 236 and 239 nm using three-dimensional ion imaging

The electronically excited states HCl(*)(E,upsilon(')=0,J(')=0) and HCl(*)(V,upsilon(')=12,J(')=0) have been prepared by two-photon resonant absorption of ground state HCl via Q(0) transitions at 238.719 and at 236.000 nm, respectively. The consequent one-or two-photon excitation at the same wavelength results in the production of H(+), Cl(+), and HCl(+) ions. The speed distributions and anisotropy parameters beta for these ions have been determined by three-dimensional photo-fragment ion imaging based on a position-sensitive delay-line anode assembly. Several results are presented: first, we measured velocity (speed and angle) distributions for HCl(+) due to the electron recoil in the photoionization of HCl(*). Such distributions give information on the photoionization process and on the vibrational distribution of HCl(+) after the laser pulse. Second, the measured beta parameters for Cl(+) and H(+) distributions give information on the symmetries of the upper states in the one-photon photoexcitation of HCl(*). Third, the measured speed distributions for H(+) help to understand the mechanism of the photodissociation of HCl(+) ions.     

Imaging the photodissociation of CH3SH in the first and second absorption bands: the CH3(X2A1)+SH(X2Pi) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).     

Ab initio potential energy surfaces, total absorption cross sections, and product quantum state distributions for the low-lying electronic states of N(2)O

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N(2)O (X (1)A('), 2 (1)A('), 3 (1)A('), 1 (1)A("), 2 (1)A(") and 3 (1)A(")) have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociation cross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck-Condon region the potential energy curves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N(2)O initially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [(1)Sigma(-)(1 (1)A(")), (1)Delta(2 (1)A(')), and (1)Delta(2 (1)A("))] have zero transition dipole moments from the ground state [(1)Sigma(+)(1 (1)A('))] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The (1)Delta(2 (1)A('))<--X (1)Sigma(+)((1)A(')) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound (1)Delta(2 (1)A(")) state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.     

High-resolution slice imaging of quantum state-to-state photodissociation of methyl bromide

The photodissociation of rotationally state-selected methyl bromide is studied in the wavelength region between 213 and 235 nm using slice imaging. A hexapole state selector is used to focus a single (JK=11) rotational quantum state of the parent molecule, and a high speed slice imaging detector measures directly the three-dimensional recoil distribution of the methyl fragment. Experiments were performed on both normal (CH(3)Br) and deuterated (CD(3)Br) parent molecules. The velocity distribution of the methyl fragment shows a rich structure, especially for the CD(3) photofragment, assigned to the formation of vibrationally excited methyl fragments in the nu(1) and nu(4) vibrational modes. The CH(3) fragment formed with ground state Br((2)P(3/2)) is observed to be rotationally more excited, by some 230-340 cm(-1), compared to the methyl fragment formed with spin-orbit excited Br((2)P(1/2)). Branching ratios and angular distributions are obtained for various methyl product states and they are observed to vary with photodissociation energy. The nonadiabatic transition probability for the (3)Q(0+)-->(1)Q(1) transition is calculated from the images and differences between the isotopes are observed. Comparison with previous non-state-selected experiments indicates an enhanced nonadiabatic transition probability for state-selected K=1 methyl bromide parent molecules. From the state-to-state photodissociation experiments the dissociationenergy for both isotopes was determined, D(0)(CH(3)Br)=23 400+/-133 cm(-1) and D(0)(CD(3)Br)=23 827+/-94 cm(-1).     

A velocity map imaging study of the one and two photon dissociations of state-selected DCl+ cations

DCl(+)(X (2)Pi(32),v(+")=0) cations have been prepared by 2+1 resonance enhanced multiphoton ionization, and their subsequent fragmentation following excitation at numerous wavelengths in the range of 240-350 nm studied by velocity map imaging of the resulting Cl(+) products. This range of excitation wavelengths allows selective population of A (2)Sigma(+) state levels with all vibrational (v(+')) quantum numbers in the range 0< or =v(+')< or =15. Image analysis yields wavelength dependent branching ratios and recoil anisotropies of the various D+Cl(+) ((3)P(J), (1)D, and (1)S) product channels. Levels with 10< or =v(+')< or =15 have sufficient energy to predissociate, forming D+Cl(+)((3)P(J)) products with perpendicular recoil anisotropies-consistent with the A (2)Sigma(+)<--X (2)Pi parent excitation and subsequent fragmentation on a time scale that is fast compared with the parent rotational period. Branching into the various spin-orbit states of the Cl(+)((3)P(J)) product is found to depend sensitively upon v(+') and, in the case of the v(+')=13 level, to vary with the precise choice of excitation wavelength within the A (2)Sigma(+)<--X (2)Pi(13,0) band. Such variations have been rationalized qualitatively in terms of the differing contributions made to the overall predissociation rate of DCl(+)(A,v(+')) molecules by coupling to repulsive states of (4)Pi, (4)Sigma(-), and (2)Sigma(-) symmetries, all of which are calculated to cross the outer limb of the A (2)Sigma(+) state potential at energies close to that of the v(+')=10 level. Cl(+)((3)P(J)) fragments are detected weakly following excitation to A (2)Sigma(+) state levels with v(+')=0 or 1, Cl(+)((1)D) fragments dominate the ion yield when exciting via 2< or =v(+')< or =6 and via v(+')=9, while Cl(+)((1)S) fragments dominate the Cl(+) images obtained when exciting via levels with v(+')=7 and 8. Analysis of wavelength resolved action spectra for forming these Cl(+) ions and of the resulting Cl(+) ion images shows that (i) these ions all arise via two photon absorption processes, resonance enhanced at the one photon energy by the various A(v(+')<10) levels, (ii) the first A (2)Sigma(+)<--X (2)Pi absorption step is saturated under the conditions required to observe significant two photon dissociation, and (iii) the final absorption step from the resonance enhancing A(v(+')) level involves a parallel transition.     

The study of the D(') (1)Pi(u) state of H(2): transition probabilities from the ground state, predissociation yields, and natural linewidths

The absorption spectrum of the H(2) molecule was studied at high resolution in the 81-72 nm spectral range. A detailed analysis of the D(') (1)Pi(u)-->X (1)Sigma(g) (+) electronic band system is reported. In the spectrum, more than 70 new lines were assigned. For wavelengths longer than 75 nm, the D(') (1)Pi(u) (+) and (1)Pi(u) (-) components show a clearly different behavior: Tauhe (1)Pi(u) (+) one dissociates into H(1s)+H(n=2) whereas the (1)Pi(u) (-) one leads to molecular fluorescence. For shorter wavelengths, both components are predissociated into H(1s)+H(n=3). The predissociation yields, the dissociation widths, and the absolute values of the transition probabilities were measured over the vibrational progression from v(')=3 to 17, i.e., up to the dissociation limit. The comparison between these absolute transition probabilities and the values calculated in the adiabatic and nonadiabatic approximations demonstrates clearly the importance of nonadiabatic couplings.     

Observation of a new phosphorus-containing reactive intermediate: electronic spectroscopy and excited-state dynamics of the HPBr free radical

The A (2)A(')-X (2)A(") electronic spectra of jet-cooled HPBr and DPBr have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of PBr(3) and H(2)/D(2). Laser-induced fluorescence and single vibronic level emission spectra gave the bending and P-Br stretching frequencies in the ground and excited states of both isotopomers. Rotational analyses of the HPBr and DPBr 0(0) (0) bands showed small spin splittings characteristic of a doublet-doublet transition of an asymmetric-top molecule. From the ground- and excited-state rotational constants, effective (r(0)) structures were derived with r(")(PH)=1.4307(86) A, r(")(PBr)=2.2021(9) A, and theta(")=95.2(8) degrees, and r(')(PH)=1.434(31) A, r(')(PBr)=2.1669(26) A, and theta(')=115.5(16) degrees . In a few favorable cases, further hyperfine splitting of the spin-rotation energy levels has been observed, due to the excited-state Fermi contact interaction of the unpaired electron with the spin magnetic moment of the (31)P nucleus, with a(F) (')=0.064(9) cm(-1) for HPBr. Fluorescence depletion spectroscopy and lifetime measurements indicate that higher vibrational levels of the A (2)A(') state are predissociated by a X (2)A(") dissociative continuum. CCSD(T)/aug-cc-pVTZ calculations predict that the most likely dissociation process is HPBr (X (2)A("))-->PH((3)Sigma(-))+Br((2)P(u)).     

1 Pi<--X1 Sigma+ band systems of jet-cooled ScCo and YCo

Rotationally resolved resonant two-photon ionization (R2PI) spectra of ScCo and YCo are reported. The measured spectra reveal that these molecules possess ground electronic states of (1)Sigma(+) symmetry, as previously found in the isoelectronic Cr(2) and CrMo molecules. The ground state rotational constants for ScCo and YCo are B(0)(")=0.201 31(22) cm(-1) and B(0) (")=0.120 96(10) cm(-1), corresponding to ground state bond lengths of r(0) (")=1.812 1(10) A and r(0) (")=1.983 0(8) A, respectively. A single electronic band system, assigned as a (1)Pi<--X (1)Sigma(+) transition, has been identified in both molecules. In ScCo, the (1)Pi state is characterized by T(0)=15,428.8, omega(e)(')=246.7, and omega(e)(')x(e)(')=0.73 cm(-1). In YCo, the (1)Pi state has T(0)=13 951.3, omega(e)(')=231.3, and omega(e)(')x(e) (')=2.27 cm(-1). For YCo, hot bands originating from levels up to v(")=3 are observed, allowing the ground state vibrational constants omega(e)(")=369.8, omega(e)(")x(e)(")=1.47, and Delta G(12)(")=365.7 cm(-1) to be deduced. The bond energy of ScCo has been measured as 2.45 eV from the onset of predissociation in a congested vibronic spectrum. A comparison of the chemical bonding in these molecules to related molecules is presented.     

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of 2,5-dimethylpyrrole

The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244 < lambda(phot) < 282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at lambda(phot)> or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 30 530 +/- 100 cm(-1). Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N-H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 1(1)A(2) potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)<-- X(1)A(1) absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N-H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N-H bond lengths is proposed as a further mechanism for modest translational --> vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths approximately 250 nm, and PTS spectra recorded at lambda(phot)< or = 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 1(1)A(2) state.     

State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Slice imaging of nitric acid photodissociation: The O(1D) + HONO channel

We report an imaging study of nitric acid (HNO(3)) photodissociation near 204 nm with detection of O((1)D), one of the major decomposition products in this region. The images show structure reflecting the vibrational distribution of the HONO coproduct and significant angular anisotropy that varies with recoil speed. The images also show substantial alignment of the O((1)D) orbital, which is analyzed using an approximate treatment that reveals that the polarization is dominated by incoherent, high order contributions. The results offer additional insight into the dynamics of the dissociation of nitric acid through the S(3) (2 (1)A(')) excited state, resolving an inconsistency in previously reported angular distributions, and pointing the way to future studies of the angular momentum polarization.     

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.     

二溴代烷烃的紫外光解动力学研究

利用飞行时间质谱装置研究了234和267nm激光作用下二溴甲烷、二溴乙烷、二溴丙烷和二溴丁烷分子的光解离过程．研究表明二溴代烷烃分子在紫外激光的作用下主要是断裂C—Br键解离出一个Br原子,并且存在两种可能的布居：基态Br(^2P3/2^0)和激发态Br^*(^2P1／2^0)．通过共振增强多光子电离技术探测两种光解产物布居的分支比．对比得到了分子构型对称性不同的二溴代烷烃的分支比,提出了两种假设的光解离模型．     

Doubly excited 2 1Delta g state of Na2

The doubly excited valence (3p+3p) 2 (1)Delta(g) state of Na(2) is experimentally observed by using optical-optical double resonance spectroscopy. A single line Ar(+) laser (a total of nine lines) was used to pump the sodium dimers from thermally populated ground state X (1)Sigma(g) (+) to the intermediate B (1)Pi(u) state. Then, a single mode Ti:sapphire laser was used to probe the doubly excited 2 (1)Delta(g) state. Violet fluorescence emitted from the highly excited states (mainly 2 (3)Pi(g) or 3 (3)Pi(g) states which are transferred from 2 (1)Delta(g) state via collision) to the a (3)Sigma(u) (+) state was monitored by a filtered photomultiplier tube (PMT). A total of 582 rovibrational levels of 2 (1)Delta(g) state were observed, identified, and assigned to the vibrational and rotational quantum numbers in the range of 0< or =v< or =28 and 11< or =J< or =99, respectively. The absolute vibrational quantum number assignment was verified by comparing the totally resolved fluorescence with the calculated Franck-Condon factors between 2 (1)Delta(g) state and B (1)Pi(u) state. Dunham coefficients and Rydberg-Klein-Rees potential curve were derived from these observed quantum levels. The primary molecular constants of Na(2) 2 (1)Delta(g) state are T(e)=32 416.759(15) cm(-1), omega(e)=124.8484(36) cm(-1), B(e)=0.119 158(3) cm(-1), and R(e)=3.508 20(5) A.     

The photodissociation reaction dynamics of CF(3)I at 304 nm ((3)Q(0+), (1)Q(1)<--(3)Q(0+), and (3)Q(1))

The photodissociation of CF(3)I at 304 nm has been studied using long time-delayed core-sampling photofragment translational spectroscopy. Due to its capability of detecting the kinetic energy distribution of iodine fragments with high resolution, it is able to directly assign the vibrational state distribution of CF(3) fragments. The vibrational state distributions of CF(3) fragments in the I(*)((2)P(12)) channel, i.e., (3)Q(0+) state, have a propensity of the nu(2) (') umbrella mode with a maximum distribution at the vibrational ground state. For the I((2)P(32)) channel, i.e., (1)Q(1)<--(3)Q(0+), the excitation of the nu(2) (') umbrella mode accounts for the majority of the vibrational excitation of the CF(3) fragments. The 1 nu(1) (') (symmetric CF stretch) +nnu(2) (') combination modes, which are associated with the major progression of the nu(2) (') umbrella mode, are observed for the photodissociation of CF(3)I at the I channel, i.e., (3)Q(1) state. The bond dissociation energy of the CI bond of CF(3)I is determined to be D(0)(CF(3)-I)相似文献