ALBERT

Notes: The rovibrational state distribution of the nascent NH2(A˜ 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (6000–13 000 cm−1) of the NH2(A˜ 2A1→X˜ 2B1) near infrared emission spectrum obtained by time-resolved Fourier transform infrared emission spectroscopy. The NH2(A˜) fragments are observed to be formed predominantly in their zero-point vibrational level, with substantial rotational excitation about their a-inertial axis up to the limit of the available energy, ∼3150 cm−1, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH3 A˜ state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out-of-plane bending vibrational motion introduced in the parent molecule by the photoexcitation process.

Notes: The photodissociations of jet-cooled IBr and Br2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(2P3/2o)+Br(2P3/2o) and Br(2P3/2o)+Br*(2P1/2o) products from parent molecules in their v″=0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v″=1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v″=0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br2. © 2002 American Institute of Physics.

Notes: The photodissociation of jet-cooled IBr molecules has been investigated at numerous excitation wavelengths in the range 440–685 nm using a state-of-art ion imaging spectrometer operating under optimal conditions for velocity mapping. Image analysis provides precise threshold energies for the ground, I(2P3/2)+Br(2P3/2), and first excited [I(2P3/2)+Br(2P1/2)] dissociation asymptotes, the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of excitation wavelength. Such experimental data have allowed mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorptions and thus deconvolution of the separately measured (room temperature) parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+) and 1Π(1) excited states of IBr. Such analyses of the continuous absorption spectrum of IBr, taken together with previous spectroscopic data for the bound levels supported by the A and B state potentials, has allowed determination of the potential energy curves for, and (R independent) transition moments to, each of these excited states. Further wave packet calculations, which reproduce, quantitatively, the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies, serve to confirm the accuracy of the excited state potential energy functions so derived and define the value (120 cm−1) of the strength of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry. © 2001 American Institute of Physics.

Notes: The Lyman-α (λH=121.6 nm) photodissociation of both H2S and D2S has been reinvestigated using the experimental technique of H/D atom photofragment translational spectroscopy. Their total kinetic energy release profiles consist of two distinct components. The first, which is highly structured, is assigned to two body dissociation to H/D(2S)+SH/SD(A 2Σ+) fragments, with the latter formed in a range of rovibrational states. By assigning these various levels the dissociation energy of D2S (measured relative to the lowest rovibrational level of the products) is determined to be D0(D-SD)≥31 874±22 cm−1. The second contribution, which is broad and relatively unstructured, is modeled in terms of two likely fragmentation pathways; secondary predissociation of SH/SD(A 2Σ+) partner fragments associated with the structured contour, and primary three-body dissociation to 2H/D(2S)+S(1D) atomic products. The presented data allow determination of the kinetic energy-dependent anisotropy parameter (β), which is positive over both profiles. This indicates a preferentially parallel distribution of H/D atom recoil velocities about the laser polarization axis. These data are presented in tandem with ab initio and classical trajectory calculations which seek to explain the lack of branching to ground state H/D+SH/SD(X 2ΠΩ) molecular products. The analogous channel is important in the Lyman-α dissociation of the lighter homologue, H2O. © 2001 American Institute of Physics.

Notes: The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the "alignment-free" recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis. © 2002 American Institute of Physics.

Notes: The photochemistries of HCN and DCN at the H(D) Lyman-α wavelength have been reinvestigated using the technique of H(D) Rydberg atom time-of-flight spectroscopy, with angular resolution of the H/D atom signal about the polarization vector of the photolysis radiation. In the case of HCN photodissociation, the previous assignment of substantial branching to H+CN(A 2Π)v=0 products is confirmed. Analysis of the profile taken under parallel polarization of the Lyman-α radiation relative to the time of flight axis reveals additional structure attributable both to a progression in CN(A) products with high rovibrational excitation (v=4–9, with N∼26–41, for all v), and to various rotationally excited levels associated with CN(B 2Σ+)v=0,1. From these various assignments an improved value for the dissociation energy, D0(H–CN)=43 710±70 cm−1, is obtained. The determined β parameter, which is a measure of the angular part of the photofragment velocity distribution about the polarization vector of the photolysis radiation, shows an increasingly parallel distribution of H atom velocities with increasing CN internal energy. DCN photolysis at the D Lyman-α wavelength yields both CN(A)v=0 and a range of rovibrationally excited CN(A) products but, in contrast to HCN, no significant branching to CN(B) products is observed. The corresponding β parameter, which is found to be relatively invariant with CN internal energy, indicates a near limiting perpendicular distribution of D atom velocities about the photolysis radiation. These results are interpreted with reference to the available ab initio calculated potential energy surfaces of A′ and A″ symmetry, and the relative propensities for excitation to, and the likely dynamics on, these various excited states is discussed. © 2000 American Institute of Physics.

Notes: The near ultraviolet photolysis of jet-cooled acetylene molecules has been further investigated using the technique of H atom photofragment translational spectroscopy. Analysis of the rotational structure evident in the total fragment kinetic energy spectrum yields a precise value for the C–H bond dissociation energy: D0(HCC–H)=46 074±8 cm−1 (551.2±0.1kJ mol−1).

Notes: The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A‘−X˜ 1A') absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm−1. The resulting CH3S(X˜) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm−1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X˜) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A‘−X˜ 1A') absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X˜) fragments yielding thioformaldehyde (H2CS), primarily in its A˜ 1A2 excited electronic state. Symmetry arguments provide an explanation for this specific electronic branching in the near ultraviolet photolysis of CH3S fragments.

Notes: The photodissociation of jet-cooled HCCH molecules following excitation to their S1 state has been investigated further, at a number of wavelengths in the range 205–220 nm, using the H atom photofragment translational spectroscopy (PTS) technique. Analysis of the rovibrational structure evident in the total kinetic energy release (TKER) spectra so obtained confirms previous reports that the resulting C2H(X˜) fragments are formed in most (if not all) of the v2 bending vibrational levels permitted by energy conservation, and that there is a clear preference for populating those states in which the axial projection of this vibrational angular momentum is maximized (i.e., states with l=v2). The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths (e.g., λphot=205.54 nm) shows bimodal rotational distributions, and a marked anisotropy—especially in the case of those H atoms that are formed in association with C2H(X˜) fragments carrying little rotational excitation. Two competing dissociations mechanisms have been identified. Our discussion of these observations is guided by the recent ab initio calculations of Cui and Morokuma [Chem. Phys. Lett. 272, 319 (1997)]. Channel I conforms to their proposal that the S1 molecules reach the H+C2H(X˜) asymptote as a result of sequential nonadiabatic couplings via the T3, T2, and T1 potential energy surfaces. The product energy disposal at the longest excitation wavelengths is rationalized in terms of the forces acting as the dissociating molecule traverses a late barrier in the C–H exit channel on the T1 surface, while the propensity for populating states with l=v2 reflects the importance of parent torsional motion in promoting the S1→T3, T3→T2, and T2→T1 surface couplings. The population of low rotational states with high recoil anisotropy at shorter excitation wavelengths is ascribed to channel II, involving a direct nonadiabatic transition from S1 to T1 for a structure with one near linear CCH angle. In contrast to channel I, there is no extensive torsional motion and the anisotropy of the initial excitation is retained through to fragmentation. Excitation of the ν1′ mode of HCCH enhances the branching to channel II. © 1998 American Institute of Physics.

Notes: The spectroscopy and predissociation dynamics of the A˜ 1A′′ state of HNO have been investigated by measurement of line positions and lifetime broadened linewidths in the cavity ring-down (CRD) spectrum. CRD spectroscopy is a technique better suited to studies of molecular predissociation than methods such as laser induced fluorescence in cases where the excited state dissociation lifetime is short compared to its fluorescence lifetime. The CRD spectrum extends well beyond the dissociation limit (we have identified transitions to rotational states lying up to 2400 cm−1 above the dissociation limit of 16450 cm−1). The lifetime-dependent Lorentzian components of the line shapes of numerous rovibrational features of the A˜ 1A′′–X˜ 1A CRD absorption spectrum have been deconvoluted from the Doppler and laser line profiles to obtain lifetimes and predissociation rates for individual |v1v2v3〉|JK〉 states. Here, the labels v1, v2, and v3 denote the number of quanta of the N–H stretch, N(Double Bond)O stretch, and H–N–O bending vibrations, respectively. We have measured line broadening (of up to 0.3 cm−1) in transitions to six vibronic states above the predissociation threshold (the 100 and 020 states, for which the higher K levels are above the dissociation limit, and the 101, 030, 110, and 111 states). For three substates (100 K=5, 101 K=1 and 110 K=4) strongly J-dependent transition linewidths are seen. The 100 K=5 and 101 K=1 substates show maximum transition linewidths midway through the observed spectral transitions while the linewidths for transitions involving the 110 K=4 substate increase with J. Linewidths also generally increase with K. Some lines involving the 100 K=5 state are markedly asymmetric. Linewidths for transitions to states having excitation of the bending mode (the 101 and 111 states) are larger than those for which v3=0. These observations clarify the predissociation mechanism suggested by previous absorption and LIF studies. We attribute the primary predissociation mechanism to a-axis Coriolis coupling of A˜ state levels to discrete quasibound highly vibrationally excited levels of the ground state which in turn are coupled to the electronic ground state continuum corresponding to dissociation to H(2S)+NO(X 2Π). Predissociation of A˜ state levels with K=0 is probably caused by b-axis Coriolis coupling to such quasibound levels of the ground state. The variation of predissociation rates with J and K for the A˜ 110 K=4, 5, and 6 substates cannot be accounted for by this mechanism and we propose the onset of predissociation to the continuum of the a˜ 3A′′ state. Interpretation of our experimental data is assisted by calculations performed using the potential energy surfaces of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)]. © 1997 American Institute of Physics.