H. Reisler

Reconstruction of Abel-transformable images: The Gaussian basis-set expansion Abel transform method

In this article we present a new method for reconstructing three-dimensional (3D) images with cylindrical symmetry from their two-dimensional projections. The method is based on expanding the projection in a basis set of functions that are analytical projections of known well-behaved functions. The original 3D image can then be reconstructed as a linear combination of these well-behaved functions, which have a Gaussian-like shape, with the same expansion coefficients as the projection. In the process of finding the expansion coefficients, regularization is used to achieve a more reliable reconstruction of noisy projections. The method is efficient and computationally cheap and is particularly well suited for transforming projections obtained in photoion and photoelectron imaging experiments. It can be used for any image with cylindrical symmetry, requires minimal user’s input, and provides a reliable reconstruction in certain cases when the commonly used Fourier–Hankel Abel transform method fails.

The 266 nm photolysis of ICN: Recoil velocity anisotropies and nascent E,V,R,T excitations for the CN+I(2P3/2) and CN+I(2P1/2) channels

We report the detection of nascent CN(X 2Σ+, v‘=0) following the 266 nm photodissociation of 300 K ICN, using sub‐Doppler resolution laser‐induced fluorescence, and polarized photolysis and probe lasers. When monitoring a particular CN internal state, the translational energies of the I+CN and I*+CN channels differ by the iodine spin‐orbit splitting 7603 cm−1. This is used to determine the separate contributions from each channel. For I+CN, high N‘ are selectively produced, with little population below N‘=20 (〈Erot〉 =3300±300 cm−1), whereas the I*+CN channel is associated with a distribution peaked sharply at low N‘(〈Erot〉 =355±35 cm−1). It is clear that the low and high N‘ derive from linear and bent exit channel geometries, respectively. The spatial anisotropy is high (βI =1.3±0.2; βI* =1.6±0.2) and initial excitation is via a parallel transition(s), probably to a state which begins correlating with I*+CN in the linear configuration. Nascent spin‐rotation states (F1 and F2) are also resolved for each ch...

Nascent product excitations in unimolecular reactions: The separate statistical ensembles method

The unimolecular reaction of a vibrationally excited molecule having low rotational excitation often leads to nascent products in which the vibrational degrees of freedom appear ‘‘hotter’’ than the rotation, translation (R,T) degrees of freedom. We show that this can derive from parent vibrations being ‘‘hot’’ while parent rotations remain ‘‘cold,’’ since the parentage of product vibration is parent vibration, while product R,T excitations are obtained from parent vibration as well as rotation. Calculations are performed for reactions having loose transition states and no reverse barriers, in which an ensemble of 3N–6 degrees of freedom are used to equilibrate parent vibrations, thereby providing a statistical distribution of product vibrational excitations. For each set of product vibrational states, all R,T excitations are then apportioned statistically using the phase space theory of unimolecular reactions (PST). The results indicate that for those energies above reaction threshold (E‡) which exceed th...

The monoenergetic unimolecular reaction of expansion‐cooled NO2: NO product state distributions at excess energies 0–3000 cm−1

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion‐cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observ...

NCNO → CN+NO: Complete NO(E, V, R) and CN(V, R) nascent population distributions from well‐characterized monoenergetic unimolecular reactions

We report detailed vibrational, rotational, and electronic (V, R, E) distributions of nascent NO(X 2Π) deriving from monoenergetic unimolecular reactions of jet‐cooled NCNO. Excitation is via the A 1A″ ← X 1A’ system above dissociation threshold (17 085±5 cm−1), and vibrational predissociation occurs following radiationless decay of the initially excited A 1A″ state. These results are combined with data on the corresponding CN(X 2Σ+) nascent V, R distributions, thereby providing a complete description of the energy partitioning into the various degrees of freedom of both products. The data presented here support our previous conclusion that dissociation is ‘‘statistical.’’ All the V, R distributions of both products can be predicted accurately using a modification of the phase space theory of unimolecular reactions (PST), which we call the separate statistical ensembles (SSE) method; it is expected that this method will have quite general applicability. NO spin‐orbit excitation is ‘‘cold’’ relative to ...

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2☆

Abstract The 300 K reactions of O 2 with C 2 (X 1 Σ + g ), C 2 (a 3 Π u ), C 3 (X 1 Σ + g ) and CN(X 2 Σ + ), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C 2 H 3 CN in the presence of O 2 , CO molecules in the a 3 Σ + , d 3 Δ i , and e 3 Σ − states were identified, as well as CH(A 2 Δ) and CN(B 2 Σ + ) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C 2 (X 1 Σ + g , a 3 Π u ) + O 2 → CO(X 1 Σ + + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ) by O 2 were determined both from laser induced fluorescence of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10 −12 cm 3 molec −1 s −1 . The rate coefficient of the reaction of C 3 with O 2 , which was determined using the IR MPD of allene as the source of C 3 molecules, is −14 cm 3 molec −1 s −1 . In addition, we find that rate coefficients for C 3 reactions with N 2 , NO, CH 4 , and C 3 H 6 are all −14 cm 3 molec −1 s −1 . Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10 −11 cm 3 molec −1 s −1 . Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O 2 produces NCO in vibrationally excited states. Radiative lifetime of the Ā 2 Σ state of NCo and the Ā 1 Π u (000) state of C 3 are reported.

The monoenergetic vibrational predissociation of expansion cooled NCNO: Nascent CN(V,R) distributions at excess energies 0–5000 cm−1

We report detailed vibration, rotation distributions for nascent CN(X 2∑+), following the one‐photon photodissociation of expansion cooled NCNO via π*←n excitation throughout the region 450–585 nm. At the observed threshold for dissociation (585.3 nm), >90% of the CN product is in v″=0, N″=0, with the remainder in N″=1, corresponding to 〈Erot〉 <0.4 cm−1. CN(X 2∑+, v″=0) rotational distributions are obtained at many photolysis wavelengths and rotational levels are observed up to, but never above, the limit imposed by energy conservation: [B″vN″(N″+1)]<E p−D0(v″), where D0(v″) is the dissociation energy to produce CN(X 2∑+,v″) and Ep is the photon energy. CN(X 2∑+,v″=1) and CN(X 2∑+,v″=2) thresholds are observed at photolysis wavelengths which correspond exactly to Ep−D0(v″=1) and Ep−D0(v″=2). These observations can only be reconciled with a vibrational predissociation mechanism and spectroscopic observations suggest that this occurs following internal conversion to the ground state surface. With Ep−D0(v″) ...

Kinetics of free radicals generated by IR laser photolysis. IV. Intersystem crossings and reactions of C2(X 1Σ+g) and C2(a 3Πu) in the gaseous phase

Rate coefficients (300 K) for the removal of C2(X 1Σ+g) and C2(a 3Πu), hereafter referred to as 1C2 and 3C2 respectively, by H2, NO, and a number of hydrocarbons are reported as well as rate coefficients for intersystem crossing between 1C2 and 3C2 induced by collisions with N2, CO2, CF4, Ar, Kr, and Xe. C2 molecules are produced via ir photolysis of C2H3CN or C2HCl3, and their concentrations are monitored by laser induced fluorescence. We find that collisionally induced intersystem crossing is significant only when it is spin allowed or involves heavy collision partners (e.g., Kr, Xe). 1C2 reacts more rapidly with NO than does 3C2, and excited CN molecules in the A and B states are formed predominantly in reactions of 3C2. 1C2 reactions result (mainly) in ground state CN, as expected from adiabatic state correlations. Radiationless transitions between the X and B states of CN, induced by collisions with Ar, are observed. It is suggested that both 1C2 and 3C2 are removed by hydrocarbons mainly via chemica...

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

This review discusses low-lying (n = 3) Rydberg electronic states of molecules and radicals, in particular those that result in mixed configurations and participate in non-adiabatic interactions. Rydberg-valence interactions are ubiquitous in molecules and radicals and have important effects on potential energy surface shapes, spectroscopy, and dissociation dynamics. The review emphasises selected properties that are affected by these interactions and illustrates them with several examples, mostly involving free radicals, in which collaboration between experimental and theoretical investigations has been crucial for understanding. Specifically, the cases of substituted methyl, vinyl, and hydroxyalkyl radicals are described, as well as Rydberg states of diazomethane and the ethyl radical. The issue of triple conical intersections is also briefly discussed.

The kinetics of free radicals generated by IR laser photolysis. I. Reactions of C2(a 3Πu) with NO, vinyl cyanide, and ethylene

C2(a 3Πu) molecules are produced by multiple photon dissociation of either vinyl cyanide or ethylene with the focused output from a CO2 TEA laser. Their reaction with NO is reported in this paper. The total quenching rate coefficient of C2(a 3Πu) by NO is determined both from laser induced fluorescence of C2(a 3Πu) and from the time resolved chemiluminescence from CN(B 2Σ+) and CN(A 2Π) which are formed in the reaction, and is (7.5±0.3)10−11 cm3 molecule−1 sec−1. The vibrational energy disposal within the B 2Σ+ and A 2Π states of CN is estimated from the resolved bands of the CN(B 2Σ+→X 2Σ+) and CN(A 2Π→X 2Σ+) emission systems. Vibrational levels of CN(B 2Σ+) up to at least v′=5 but lower than v′=10 are excited, accompanied by high rotational excitation. CN in the A 2Π state is excited to at least the 16th vibrational level. The reaction populates the A 2Π state about 7 times more efficiently than it does the B 2Σ+ state. The reaction mechanism is discussed, and it is concluded that most of the electronic...