I. Nadler

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...

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 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″) ...

Energy disposal in the laser photodissociation of ICN and BrCN at 300 K and in a free jet expansion

Abstract Photodissociation of ICN and BrCN via the long-wavelength portions of their A continua leads to CN(X 2 Σ + ) rovibrational excitation which can characterize the excited potential surfaces. Parent rovibrational excitation strongly influences the observed CN state distributions.

The 540–900 nm photodissociation of 300 K NCNO: One‐ and two‐photon processes

The laser photodissociation of 300 K NCNO throughout the region 540–900 nm is reported, and both 1‐ and 2‐photon processes are discussed. By monitoring CN fragments produced via the 1‐photon process, we show that with photolysis wavelengths >592 nm, dissociation occurs predominantly by exciting NCNO ‘‘hot bands.’’ At shorter photolysis wavelengths, dissociation from the ground vibrational state of NCNO is observed as well, but the contributions from hot bands are still manifest in high CN rotational levels which are energetically inaccessible from the ground state (D0=48.8 kcal mol−1). Energy distributions in the CN fragments were determined for excess energies up to 1800 cm−1, and are in agreement with phase space theory calculations and a vibrational predissociation mechanism. In addition, throughout the region 620–900 nm, stepwise two‐photon photodissociation proceeds using the A 1A″ state as a gateway, and results in rotationally and vibrationally ‘‘hot’’ CN fragments. The hot CN fragment yield vs ph...

Near threshold photodissociation of expansion cooled NCNO: Nascent CN(X2Σ+) without internal excitation

Abstract The dissociation of expansion cooled NCNO following A 1 A″  X 1 A′ excitation transpires via vibrational predissociation, and the CN(X 2 Σ) fragments are extremely cold near reaction thereshold. Only υ″ = O is produced, and 90% of the CN is in N ″ = O corresponding to ( E internal ) −1 . A revised value for the NCNO dissociation energy ( D o = 17085 ± 10 cm −1 ) is reported.

The rotationally resolved à 1A″←X̃ 1A′ spectrum of expansion cooled NCNO: Vibrational fundamentals, rotational constants, and perturbations

The laser excitation spectrum (A 1A″←X 1A′, T′0 =11 339 cm−1) of expansion cooled nitrosyl cyanide (NCNO) has been observed at rotational resolution using the technique of two‐photon photodissociation followed by laser induced fluorescence detection of the CN fragment. All six excited state fundamental vibrations are assigned. The A 1A″ state geometry is derived from the measured rotational structure, and corresponds to a planar, trans conformation. Most of the bands are strongly perturbed by ground state levels, and there is strong evidence suggesting that A 1A″/X 1A′ mixing leads to the vibrational predissociation.

Simultaneous one‐ and two‐photon processes in the photodissociation of NCNO using a tunable dye laser

Photodissociation of NCNO in the region 660–732 nm shows that direct excitation to a state different than S1 is responsible for dissociation following one‐photon absorption at wavelengths <720 nm. Also, throughout the region 660–732 nm, a second photon can easily excite molecules in S1, thereby leading to efficient two‐photon photodissociation. It is possible to distinguish between the one‐ and two‐photon processes, which produce rotationally ‘‘cold’’ and ‘‘hot’’ CN radicals, respectively. (AIP)