Photofragment Translational Spectroscopy Research Articles

Photodissociation dynamics of H2S(D2S) following excitation within its first absorption continuum

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of H2S(D2S) following excitation at a number of wavelengths in its first absorption continuum. The respective bond dissociation energies were determined as D0(H–SH)=3.90±0.03 eV and D0(D–SD)=4.00±0.02 eV. The measured vibrational energy disposal in the partner ground state SH(SD) fragments and the product angular distributions support and extend the earlier results of van Veen et al. [Chem. Phys. 74, 261 (1983)], while the deduced rotational and spin–orbit state population distributions in these fragments appear to be generally consistent with the recent measurements of Weiner et al. [J. Chem. Phys. 90, 1403 (1989)]. We present a schematic adiabatic potential energy surface for the dissociative 1A″ excited state via which it is possible to rationalize virtually all existing dynamical measurements relating to this fragmentation process.

Photodissociation of C 2F 4 at 193.3 nm; the production of triplet CF 2( 3B 1)

The combination of photofragment translational spectroscopy data and the emission spectrum of nascent photoproducts shows that photodissociation of C 2F 4 at 193.3 nm can follow two decomposition pathways, both involving breakage of the CC double bond. One channel leads to two ground-state singlet CF 2( 1A 1) products, while the other channel correlates with formation of one ground-state singlet CF 2( 1A 1) fragment and one excited-state triplet CF 2( 3B 1) fragment. The angular dependence of the photodissociation products with respect to the laser polarization direction indicates that the transition dipole moment is aligned perpendicular to the CC bond.

Dissociation of cyclohexene and 1,4-cyclohexadiene in a molecular beam

Molecular-beam photofragmentation translational spectroscopy of cyclohexene and 1,4-cyclohexadiene was carried out using 193 nm and IR multiphoton excitation. At 193 nm, both the retro-Diels–Alder reaction of cyclohexene and H2 elimination from both molecules were observed in the ground electronic state, indicating the occurrence of internal conversion from the initially excited electronic states. The retro-Diels–Alder reaction is shown to be concerted up to an internal energy higher than 148 kcal/mol. Hydrogen-atom elimination was also observed for both molecules following 193 nm excitation. The H atom is eliminated from an excited state of cyclohexene and is assigned to be from a carbon adjacent to the double bond, with a corresponding C–H bond energy of 87±3 kcal/mol. It is shown that the peak of the translational energy distribution for concerted dissociation in the ground state is determined mainly by the dynamics of the potential-energy release along the reaction coordinate, and is not sensitive to either the amount of internal energy or the form of excitation.

Ultraviolet photodissociation and thermochemistry of CH2BrCH2I, CF2BrCF2I, and CF2ICF2I

Using photofragment translational spectroscopy, we have monitored the dissociation of CH2BrCH2I at 248, 266, and 308 nm, and CF2BrCF2I and CF2ICF2I at 308 nm. The primary fragments are I(2P3/2) and I(2P1/2) and the corresponding haloethyl radicals. The I(2P3/2) contribution decreases upon fluorination, but it is dominant for CH2BrCH2I at 308 nm. The electronic absorption dipole lies roughly along the C–I bond axis in every case. Stable CF2CF2Br and CF2CF2I radicals can be readily generated through photodissociation of the parent compounds, while stable CH2CH2Br could not be unambiguously observed. Upper limits to the reaction enthalpy at 0 K for CF2ICF2Br(I)→C2F4+I+Br(I) are 75±1(59±1) kcal/mol. The TOF spectra and related data suggest that there is a barrier to decomposition for CF2CF2I→C2F4+I that exceeds the C–I bond energy in the radical.

A concerted triple dissociation—the photochemistry of s-tetrazine

The method of photofragment-translational spectroscopy was used in a molecular beam study of the photochemistry of s-tetrazine. Following 1B3u ←1Ag (S1←S0 excitation) or 1B2u ←1Ag (248 nm excitation), s-tetrazine reverts to the highly vibrationally excited ground electronic state through internal conversion, then decomposes into 2HCN+N2 via concerted triple dissociation. By analyzing the experimental data with a simple model, it was found that the potential energy surface along the reaction coordinate after the transition state is very repulsive, leading to most of the available energy, on average 73.9% in the case of S1 ←S0 excitation, going into translation. The asymptotic angles between N2 and HCN with respect to the center of mass of s-tetrazine were 117.2±0.5° and 114.4±0.5° for S1 ←S0 and 248 nm excitation, respectively.

Symmetric stretch excitation of CH3 in the 193.3 nm photolysis of CH3I

New high resolution photofragment translational spectroscopy measurements on the 193.3 nm photolysis of CH3I reveal up to four quanta of ν1 C–H symmetric stretch excitation in the C–I bond fission channel. In addition these experiments show for the first time that C–H bond fission occurs at wavelengths longer than 185 nm with a quantum yield of ∼3%. The data yields upper bounds to the C–H bond dissociation energy in CH3I of 101±1 kcal/mol and for the C–I bond energy, 55.0±0.3 kcal/mol.

State selective photodissociation dynamics of à state ammonia. II

The photodissociation dynamics of à state ammonia molecules (both NH3 and ND3) has been further investigated using the technique of H(D) atom photofragment translational spectroscopy. The resulting NH2(ND2) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v′2 level excited. Dissociation from the v2=0 level of the à state yields ground state NH2(ND2) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N≂Ka appears near to statistical. In contrast, dissociation from the parent v′2=1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical trajectory calculations employing the best available ab initio potential energy surfaces for the à and X̃ states of the ammonia molecule. The energy disposal following excitation to the parent v2=2 and 3 levels is found to mimic that observed for, respectively, the v′2=0 and 1 levels. These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fragmentation of parent levels with v2≥2. The threshold energy for producing electronically excited NH2(Ã2A1) fragments is 6.02 eV [∼6.16 eV for ND2(Ã)]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed.

Molecular Beam Study of Ethyl Nitrite Photodissociation and Unimolecular Decay of Ethoxy Fragments

AbstractThe UV photodissociation of ethyl nitrite at excitation wavelenghts of 350, 248 and 193 nm was studied by photofragment translational spectroscopy in a pulsed molecular beam. At all three wavelengths the first dissociation step yields nitric oxide and ethoxy radicals, accompanied by a considerable release of translational energy. At 350 nm the ethoxy radicals are formed with internal energy low enough to be stable under collisionless conditions. At 248 and 193 nm significant fractions of the ethoxy radicals emerge with sufficient internal energy to decay spontaneously into formaldehyde and methyl radicals. At 248 nm a kinematic analysis of the photofragment arrival time distributions is consistent with a sequential dissociation mechanism.

Translational spectroscopy of HNCO photodissociation

The kinetic energy released in the photodissociation of HNCO is measured by photofragment translational spectroscopy. In this study, CO is ionized by laser ionization immediately following its ejection from HNCO. The time for the CO ions to travel the length of our spectrometer is then measured, and the kinetic energy released by the photodissociation is calculated. Changing the wavelength of the laser that ionizes the CO changes the selected internal energy and the expected change in the kinetic energy release is observed. These data confirm that the threshold for NH (a 1Δ) production is the dissociation energy.

Photodissociation of CF2BrCH2I at 248, 266, and 308 nm

The technique of photofragmentation translational spectroscopy has been used to study the photodissociation of CF2BrCH2I at excitation wavelengths of 248, 266, and 308 nm. The primary photofragments are CF2BrCH2 and either I(2P1/2) or I(2P3/2), although some C–Br bond fission does occur at 248 and 266 nm. A large fraction of the CF2BrCH2 radical product contains enough internal excitation after the primary process to undergo secondary dissociation into CF2CH2 and Br. Secondary dissociation is also observed to take place at 248 and 266 nm via absorption of a photon by the CF2BrCH2 photofragment. By observing the threshold for the spontaneous secondary dissociation process, the reaction enthalpy for CF2BrCH2I→CF2CH2+Br+I, was determined to be 67.5±2 kcal/mol, which leads to: Δ H○f,0 (CF2BrCH2I)=−92.6±2 kcal/mol. The c.m. translational energy distributions were derived for both the I(2P1/2) and I(2P3/2) dissociation channels resulting from primary C–I bond breakage. The I(2P1/2)/I(2P3/2) branching ratios are 3.3, 9.0, and 0.5 for excitation wavelengths of 248, 266, and 308 nm, respectively. The translational energy distributions also reveal that a major fraction of the CF2BrCH2 product radicals are formed with high internal energies, averaging around 50% of the excess energy. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state channels at both wavelengths proceeds via a parallel transition—i.e., the transition moment must be nearly parallel to the C–I bond.

Photodissociation of CH2ClCH2Iat 308 nm

The technique of photofragmentation translational spectroscopy has been used to study the photodissociation of CH2ClCH2I at an excitation wavelength of 308 nm. The exclusive dissociation pathway is C–I bond breakage with formation of CH2ClCH2 and I(2P3/2). The centre‐of‐mass translational energy distribution of the photofragments reveals that an average of about 50% of the excess energy appears in translation. The angular distribution of dissociation products with respect to the laser polarization indicates that the photodissociation process proceeds via a parallel transition—i.e., the transition moment must be nearly parallel to the C–I bond. Exclusive production of ground state I(2P3/2) with a parallel polarization dependence is unexpected based on the prevailing picture for alkyl iodide photodissociation.

Determination of internal energy distribution in Na2 + produced by associative-ionization collisions in crossed beams.

We report the analysis of vibrational and rotational energy of Na/sub 2/ /sup +/ ions produced by associative ionization (AI) in two processes: Na(3p)+Na(3p)..-->..Na/sub 2/ /sup +/+e and Na(3s)+Na(6p)..-->..Na/sub 2/ /sup +/+e. The nascent distributions are measured using photofragment translational spectroscopy. The results show that AI produces vibrationally excited Na/sub 2/ /sup +/ with a distribution increasing approximately linearly with vibrational level. Rotational distributions indicate a short-range opacity function for AI with a maximum impact parameter of about 2 A in both processes listed above.

Hydrogen-atom photofragment spectroscopy. Photodissociation dynamics of H2O in the B–X absorption band

The photofragmentation dynamics of H2O molecules following vacuum ultraviolet laser excitation at wavelengths within the B1A1–X1A1 absorption band have been investigated using a novel from of photofragment translational spectroscopy. Analysis of the nascent H atom time-of-flight spectra confirms the importance of the dissociation channel leading to ground-state H + OH products. In addition, it reveals that the OH(X) fragments are formed predominantly in their zero-point vibrational level with a highly excited, inverted rotational-state population distribution. A consideration of the topology of the various potential-energy surfaces sampled by the H2O molecules during this electronically non-adiabatic dissociation process suggests a likely explanation for this observed pattern of energy disposal.

Photodissociation of 1,2-chloroiodoethane at 248 and 266 nm: The enthalpy of formation of CH2ClCH2I

The technique of photofragmentation translational spectroscopy has been utilized to study the photodissociation of CH2ClCH2I at excitation wavelengths of 248 and 266 nm. The primary photofragments are C2H4Cl and either I*(2P1/2) or I(2P3/2). A fraction of the chloroethyl radical product contains enough internal excitation after the primary process to undergo secondary dissociation into C2H4 and Cl. By observing the threshold for this secondary process, the reaction enthalpy for CH2ClCH2I→C2H4+Cl+I was determined, which leads to: ΔH0f,0 (CH2ClCH2I)=−7.8±1 kcal/mol. The c. m. translational energy distribution of the photofragments was derived for both the I* and I channels, yielding I*/I branching ratios of 1.5 at 248 nm and 3 at 266 nm. The translational energy distribution also revealed that about 50% of the excess energy went into translation. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state iodine channels at both wavelengths proceeds via a parallel transition, i.e., the transition moment must be nearly parallel to the C–I bond.

Molecular beam study of the photochemistry of S1 glyoxal

The dissociation of glyoxal after excitation to low-lying vibrational levels of the first excited singlet state has been studied by using the method of molecular beam photofragment translational spectroscopy. Detection of products at masses 28, 29, and 30 provides unequivocal proof that the S/sub 1/ state of glyoxal predissociates in the absence of collisions. The product time-of-flight (TOF) spectra show the presence of three distinct dissociation channels, the major ones being formation of H/sub 2/CO + CO and the triple dissociation forming 2CO + H/sub 2/. The third minor channel is attributed to the formation of CO plus an H/sub 2/CO isomer, possibly hydroxymethylene. 2 figures.

The photodissociation of nitromethane at 193 nm

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular dissociation to NO+O, which will occur for NO2 with 72 kcal/mol or more internal energy. Evidence is presented which suggests that the NO2 produced via the minor mechanism, which releases a smaller fraction of the available energy to translation, has a large cross section for absorbing an additional photon via a parallel transition and dissociating to NO+O.

Time-of-Flight Spectroscopy of Metastable Photodissociation Fragments. N2O Dissociation in the Vacuum uv

The photodissociation of N2O involving electronically excited metastable fragments has been investigated at wavelengths ≳ 1050 Å by photofragment translational energy spectroscopy. Irradiation was carried out with light pulses of a few microseconds duration, and fragment energies were measured by means of the time-of-flight technique. Auger electron emission from metal surfaces, cesium and brass, was used to detect metastable fragments after a flight path length of 46 cm. Experiments have been made in the wavelength regions ∼ 1050–1440, ∼ 1250–1440, and ∼ 1350–1440 Å. Formation of the metastables N2(A 3Σu+) and O(1S) in the processes N2O→N2(A 3Σu+)+O(3P) and N2O→N2(X 1Σg+)+O(1S) has been observed. O(1S) appeared in a translational energy range from ∼ 3 to 1 eV, and N2(A 3Σu+) in a range from ∼ 1 to 0.1 eV. O(1S) produced electron emission upon deactivation at a cesium-cated surface, and N2(A 3Σu+) produced electron emission upon deactivation at a cesium and also a bare brass surface.