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The dissociation of energy-selected ND3+ to form ND2++D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND3+ and ND2+ give a value of 15.891±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND2+ from ND3. We have also measured the PFI-PE vibrational bands for ND3+(X;v2+=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND3+ X(0,v2+=0–3,0,0)←ND3 X(0,0,0,0). Using the 0 K AE (ND2+) and IE(ND3)=10.200±0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND3 [D0(D–ND2)=4.7126±0.0025 eV], we have determined the D0(ND2+–D), IE(ND2), and 0 K heat of formation for ND2+ to be 5.691±0.001 eV, 11.1784±0.0025 eV, and 1261.82±0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND2+), indicating that the dissociation of excited ND3+ at energies slightly above the dissociation threshold is prompt, occurring in the time scale ≤10-7 s, as observed for the NH3 system. The available energetic data for the NH2–NH2+ and NH3–NH3+ system are found to be in excellent accord with those for the ND2–ND2+ and ND3–ND3+ system after taking into account the zero-point vibrational energy corrections. This finding indicates that the thermochemical data for these two systems are reliable with well-founded error limits. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for OCS have been obtained in the energy range 15.0-19.0 eV, covering the vibronic bands of OCS[sup +] (A ²Π, B ²Σ[sup +], and C ²Σ[sup +]). The ionization energies for the formation of the ground vibrational levels of OCS[sup +] (A ²Π[sub &frac32;], A ²Π[sub ½, B ²Σ[sup +], and C ²Σ[sup +]) from the ground OCS(X ¹Σ[sup +]) state have been determined as 15.0759±0.0005 eV, 15.0901±0.0005 eV, 16.0403±0.0005 eV, and 17.9552±0.0005 eV, respectively. We have also generated the theoretical adiabatic three dimensional potential energy functions (PEFs) for OCS[sup +](A ²Π) by employing the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Using these PEFs, the spectroscopic constants and low-lying rovibronic energy levels for OCS[sup +](A ²Π) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS[sup +](A ²Π), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS[sup +](A ²Π) and OCS(X ¹&Sigma[sup +]) states, the PFI-PE spectrum for OCS[sup +](A ²Π) exhibits a long vibronic progression extending well above the OCS[sup +](B ²&Sigma[sup +]) state. On the contrary, the PFI-PE spectra for OCS[sup +] (B ²&sSigma[sup +] and C ²&Sigma[sup +]) are overwhelmingly dominated by the ground (0,0,0) bands, exhibiting only weak vibrational progressions. [ABSTRACT FROM AUTHOR]

The integrated-absorption coefficients of several hyperfine lines of the magnetic dipole allowed transition of the bromine atom, Br, center at 3685.2 cm[SUP-1] were measured, and a value for the square of the magnetic dipole transition moment of the Br atom was determined. A theoretical calculation for the magnetic dipole transition moment was also carried out using a relativistic ab initio atomic structure formulation. The theoretical value was in excellent agreement with the value predicted assuming pure LS coupling, and in reasonable agreement with experiment. The Br atom was generated in equal concentration with the cyano radical (CN) by the 193 nm photolysis of cyanogen bromine, BrCN. The CN radicals were titrated by the rapid reaction with C[SUB3]H[SUB8] to generate HCN and a small amount of HNC. Both time-resolved and frequency-scanned infrared absorption spectroscopy were used to monitor the Br, HCN, and HNC species. The photolysis of BrCN at 193 nm produced both the ground state Br([SUP2]P[SUB3/2]) and the spin-orbit excited Br([SUP2]P[SUB1/2]) atoms, and the yield for the production of Br([SUP2]P[SUB1/2]) atoms was measured to be 0.31_+ 0.01. The rate constants for the quenching of Br([SUP2]P[SUB1/2]) by BrCN and C[SUB3]H[SUB8] at 293 K were also determined. [ABSTRACT FROM AUTHOR]

Examines the electronic-vibrational transition moment for the (2,0) band of the cyano (CN) radical. Application of a direct time-resolved absorption technique; Line strength of the R[sub 1] transition; Formation of the CN radical from the pulsed-laser photolysis of cyanogen; Absorption features of CN and hydrogen cyanide.

The variation of the total integral cross section for the chemiluminescent channel with the vibrational excitation 3≤v‘≤19 is investigated using laser-induced fluorescence or coherent population transfer, a new method employing delayed interaction with two cw lasers to populate vibrationally excited levels. The cross section increases with v‘ by about 0.76% per vibrational level. The results are in agreement with the results of phase-space calculations, as well as with those from simple curve crossing models. [ABSTRACT FROM AUTHOR]

Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mean collision energy of the order of 1 meV in a supersonic seeded beam arrangement. The initial vibrational state v and rotational state j is varied in the range 4≤v≤35 and 5≤j≤25. The Na2 molecules are excited by Franck–Condon pumping near the transition region to free molecular flow and the flux of molecules which survive in the level (v,j) is monitored far downstream behind a small aperture. It is shown that elastic processes do not contribute to the observed signal. The magnitude of the cross section for the sum of all inelastic processes is as large as several hundred Å2. It is found that the vibrational motion of the molecule, which carries an energy of up to 500 meV, strongly influences the collision dynamics. The rate constant increases with v by about 35% within the range of vibrational levels studied here. Some state-to-state rate constants are also reported. The experimental results are in good agreement with data from quasiclassical trajectory calculations on a semiempirical bond-distance-dependent potential surface. The observations can be rationalized by realizing that the long range attraction increases with vibrational excitation since the polarizability of the molecule increases. Furthermore, the vibrational excitation in combination with the relatively long interaction time due to the low collision energy enhances the probability for a change of the initial quantum state of the molecule. [ABSTRACT FROM AUTHOR]

The feasability of selective and complete population transfer between atomic or molecular levels by stimulated Raman scattering with delayed pulses involving spectrally broad light with characteristics typical for pulsed lasers is investigated. In extension of previous work, the effect on the transfer efficiency of phase fluctuations and of the detuning ΔR of the laser frequencies from the two-photon resonance is analyzed. The minimum pulse energy Pmin required to achieve a transfer efficiency of nearly unity is derived analytically, with some restrictions imposed on the type of phase fluctuations. Pmin increases approximately proportional to the square of the bandwidth of the laser radiation and to the square of ΔR. The conclusions are confirmed by the results of extensive numerical calculations. These studies also reveal a high sensitivity of the transfer efficiency to the autocorrelation of the fluctuating light. [ABSTRACT FROM AUTHOR]

The dynamics of the reactions of translationally energetic H atoms with BrCN, ClCN, and (CN)[sub 2] was studied by determining both the rotational state distribution and the translational energy disposition of the CN product ground vibrational level. The reaction was carried out using H atoms with a most probable translational energy of 92 kJ mol[sup -1]. The CN radical was monitored by time- and frequency-resolved absorption spectroscopy using the CN red system (A [sup 2]Π←X [sup 2]Σ) (2,0) band near 790 nm. Sub-Doppler resolution spectroscopy was used to determine the initial translational temperature of the CN(0,J) product. The fraction of the available reaction exothermicity that appeared as CN(0) rotational energy, f[sub R], for H+XCN→HX+CN was 0.034±0.006, 0.061±0.02, and 0.13±0.007, for X=Br, Cl, and CN, respectively. Likewise, the fraction of the available reaction exothermicity that appeared as relative product translational energy, f[sub T], was 0.52±0.25, 0.52±0.20, and 0.59±0.05, for X=Br, Cl, and CN, respectively. The absolute reaction cross sections for the H+XCN→HX+CN reactions were also measured to be 0.03, 0.02, and 0.3x10[sup -16] cm[sup 2] for X=Br, Cl, and CN, respectively. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polymers are desirable optoelectronic materials, stemming from their solution processability, tunable electronic properties, and large absorption coefficients. An exciting development is the recent discovery that singlet fission (SF), the conversion of a singlet exciton to a pair of triplet states, can occur along the backbone of an individual conjugated polymer chain. Compared to other intramolecular SF compounds, the nature of the triplet pair state in SF polymers remains poorly understood, hampering the development of new materials with optimized excited state dynamics. Here, we investigate the effect of solvent polarity on the triplet pair dynamics in the SF polymer polybenzodithiophene-thiophene-1,1-dioxide. We use transient emission measurements to study isolated polymer chains in solution and use the change in the solvent polarity to investigate the role of charge transfer character in both the singlet exciton and the triplet pair multiexciton. We identify both singlet fluorescence and direct triplet pair emission, indicating significant symmetry breaking. Surprisingly, the singlet emission peak is relatively insensitive to solvent polarity despite its nominal "charge-transfer" nature. In contrast, the redshift of the triplet pair energy with increasing solvent polarity indicates significant charge transfer character. While the energy separation between singlet and triplet pair states increases with solvent polarity, the overall SF rate constant depends on both the energetic driving force and additional environmental factors. The triplet pair lifetime is directly determined by the solvent effect on its overall energy. The dominant recombination channel is a concerted, radiationless decay process that scales as predicted by a simple energy gap law.

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale