Resonance Enhanced Multiphoton Ionization Time-of-flight Research Articles

Resonance enhanced multiphoton ionization − time of flight (REMPI-TOF) detection of Br ( 2 P j ) atoms in the photodissociation of 4-bromo-2,3,5,6-tetrafluoropyridine at 234 nm: Effect of low-lying πσ* states

Abstract The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br (2P3/2) and Br*(2P1/2). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, βi, were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP.

Chlorine atom formation dynamics in the dissociation of halogenated pyridines after photoexcitation at 235 nm: A resonance enhanced multiphoton ionization-time of flight (REMPI-TOF) study

The photodissociation dynamics of halogen substituted pyridines, namely, 3-chloropyridine (ClPy) and 3-chloro-2,4,5,6-tetrafluoropyridine (ClFPy), has been studied around 235nm by detecting chlorine atoms in their spin orbit states Cl(2P3/2) and Cl∗(2P1/2) using the REMPI-TOF technique. We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine atom elimination channels. The TOF profiles for Cl and Cl∗ are found to be independent of laser polarization suggesting a zero value for β, within the experimental uncertainties. For 3-chloropyridine, the average translational energies for Cl and Cl∗ elimination channels are determined to be 3.7±1.0 and 7.0±1.5kcal/mol, respectively. Similarly, for 3-chloro-2,4,5,6-tetrafluoropyridine, the average translational energies for Cl and Cl∗ elimination channels are determined to be 8.0±1.5 and 9.0±1.5kcal/mol, respectively. The theoretical calculation suggests that the fluorine substitution increases the possibility of cross over to the π–σ∗ state from the initially prepared π–π∗ state.

Dynamics of Cl(2Pj) formation in the photodissociation of halogenated thiadiazole at 235 nm: A resonance enhanced multiphoton ionization-time of flight (REMPI-TOF) study

Abstract In one colour experiment, the photodissociation dynamics of halogen substituted thiadiazole, namely, 3,4-dichloro-1,2,5-thiadiazole, has been studied in a supersonic molecular beam around 235 nm, which prepares the molecules in its π–π* state. The study is conducted using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary chlorine atoms in their spin orbit states Cl(2P3/2) and Cl*(2P1/2). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least squares fitting method, taking into account the fragment anisotropies, βi. The TOF profiles for Cl and Cl* are found to be independent of laser polarization, i.e., the β is well characterized by a value of ∼0.0, within the experimental uncertainties. The translational energy distributions is well explained with only one component for both Cl and Cl* atom elimination channel. The average translational energies for Cl and Cl* channel are 5.2 ± 1.5 and 7.9 ± 1.5 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The experimental studies revealing the nature of translational energies distribution for Cl and Cl* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and ab initio molecular orbital (MO) theory suggest that the initially prepared π–π* state crosses over to a nearby σ* repulsive state along the C Cl bond, from where the dissociation takes place. The results also indicates the role of fast internal conversion to ground state from the initially prepared π–π* state, from where various dissociation channels occurs. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section.

Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) study of tetrachloroethylene photodissociation at 235 nm: Role of bound [formula omitted] state

In one-color REMPI-TOF experiment, the photodissociation dynamics of tetrachloroethylene has been studied by probing the chlorine atom photofragments, namely, Cl (2P3/2) and Cl∗ (2P1/2), using 2+1 REMPI scheme in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter (β), and the spin–orbit branching ratio for chlorine atom elimination channels. The β is well characterized by a value of ∼0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 17.6±1.9 and 14.0±1.7kcal/mol, while, that for the slow components are 2.2±1.0 and 3.2±1.0kcal/mol, respectively. To understand the nature of the dissociative potential energy surface involved in the chlorine atom formation channel, detailed theoretical calculations are performed using Time-dependent Density Functional Theory (TD-DFT) method.

Spectroscopy of cold LiCa molecules formed on helium nanodroplets.

We report on the formation of mixed alkali–alkaline earth molecules (LiCa) on helium nanodroplets and present a comprehensive experimental and theoretical study of the ground and excited states of LiCa. Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy and laser induced fluorescence (LIF) spectroscopy were used for the experimental investigation of LiCa from 15000 to 25500 cm–1. The 42Σ+ and 32Π states show a vibrational structure accompanied by distinct phonon wings, which allows us to determine molecular parameters as well as to study the interaction of the molecule with the helium droplet. Higher excited states (42Π, 52Σ+, 52Π, and 62Σ+) are not vibrationally resolved and vibronic transitions start to overlap. The experimental spectrum is well reproduced by high-level ab initio calculations. By using a multireference configuration interaction (MRCI) approach, we calculated the 19 lowest lying potential energy curves (PECs) of the LiCa molecule. On the basis of these calculations, we could identify previously unobserved transitions. Our results demonstrate that the helium droplet isolation approach is a powerful method for the characterization of tailor-made alkali–alkaline earth molecules. In this way, important contributions can be made to the search for optimal pathways toward the creation of ultracold alkali–alkaline earth ground state molecules from the corresponding atomic species. Furthermore, a test for PECs calculated by ab initio methods is provided.

Spectroscopy of Lithium Atoms and Molecules on Helium Nanodroplets

We report on the spectroscopic investigation of lithium atoms and lithium dimers in their triplet manifold on the surface of helium nanodroplets (HeN). We present the excitation spectrum of the 3p ← 2s and 3d ← 2s two-photon transitions for single Li atoms on HeN. The atoms are excited from the 2S(Σ) ground state into Δ, Π, and Σ pseudodiatomic molecular substates. Excitation spectra are recorded by resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectroscopy, which allows an investigation of the exciplex (Li*–Hem, m = 1–3) formation process in the Li–HeN system. Electronic states are shifted and broadened with respect to free atom states, which is explained within the pseudodiatomic model. The assignment is assisted by theoretical calculations, which are based on the Orsay–Trento density functional where the interaction between the helium droplet and the lithium atom is introduced by a pairwise additive approach. When a droplet is doped with more than one alkali atom, the fragility of the alkali–HeN systems leads preferably to the formation of high-spin molecules on the droplets. We use this property of helium nanodroplets for the preparation of Li dimers in their triplet ground state (13Σu+). The excitation spectrum of the 23Πg(ν′ = 0–11) ← 13Σu+(ν″ = 0) transition is presented. The interaction between the molecule and the droplet manifests in a broadening of the transitions with a characteristic asymmetric form. The broadening extends to the blue side of each vibronic level, which is caused by the simultaneous excitation of the molecule and vibrations of the droplet (phonons). The two isotopes of Li form 6Li2 and 7Li2 as well as isotope mixed 6Li7Li molecules on the droplet surface. By using REMPI-TOF mass spectroscopy, isotope-dependent effects could be studied.

Photodissociation Dynamics of Halogenated Thiophenes at 235 nm: A Resonance Enhanced Multiphoton Ionization-Time-of-Flight (REMPI-TOF) Study

The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.

Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) study of phosphorous oxychloride (POCl3) dissociation at 235 nm: Dynamics of Cl(2Pj) formation

In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl3 has been studied by photolyzing POCl3 and probing the chlorine atom photofragments, namely, Cl(2P3/2) and Cl∗(2P1/2) using 2+1 REMPI scheme, in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl∗ are the same, and characterized by a value of 0.0±0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 12.5±1.5 and 16.8±1.5kcal/mol, while, for the slow components, the average translational energies are 1.5±1.0 and 2.5±1.0kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl2 elimination is also observed in the photodissociation of POCl3.

Photodissociation at 193 nm of tert-Butyl Nitrite on Ag(111)

The translational and internal state properties of nitric oxide, NO, ejected by 193-nm photodissociation of tert-butyl nitrite, (CH3)3CONO, TBN, adsorbed on Ag(111) have been investigated using resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) and quadrupole mass spectrometer time-of-flight (QMS-TOF). The results are described in terms of three components depending on the number of collisions along the exit trajectory and are compared to prior work using 248- and 351-nm photons. Although 193-nm photons have higher energy, the characteristic translational energy of the collisionless NO (0.55 eV) lies between that for 248 nm (0.89 eV) and 351 nm (0.39 eV). As for gas-phase photolysis at 193 nm, the NO is rotationally cold (Jmax = 15.5), much lower than for both 248- (Jmax = 53.5) and 351-nm (Jmax = 31.5) photolysis. The NO vibrational distribution is dominated by v‘ ‘ = 1, whereas v‘ ‘ = 0 for 248 nm and v‘ ‘ = 1 and 2 for 351 nm photodissociations, respectively. Angle-resolved REMPI-TOF and QMS-TOF studies give information about the orientation of adsorbed TBN. While TBN is more stable with the C−O−NO plane perpendicular to surface and the internal O−N bond directed about 40° away from the surface normal, dosing less than a full monolayer at 80 K does not achieve this state fully. Annealing to 110 K is sufficient to realize this molecular orientation. For multilayer coverage dosed at 80 K, the data are consistent with considerable surface roughness compared to a surface annealed at 110 K.

State-resolved dynamics of NO produced by photodissociation of t-butyl nitrite on Ag(111)

The dynamics of nitric oxide, NO, produced by 248 nm photodissociation of t-butyl nitrite, (CH 3) 3CONO, adsorbed on Ag(111) has been probed using resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) and interpreted using a direct excitation and collisional relaxation model. NO TOF spectra vary with rotational state and surface coverage, submonolayer to multilayers, of t-butyl nitrite. High rotational state ( J>40.5) spectra show a sharp peak of fast NO, interpreted as collisionless ejection, along with an intermediate speed peak ascribed to NO that has collided with neighbors. The collisionless peak is evidence supporting direct excitation followed by prompt ejection of NO. As J is lowered from 50.5 to 12.5, the intermediate peak position drops from 60 to 30% of the collisionless kinetic energy. Low J (< ∼20.5) spectra show a slow peak thermalized to the surface temperature and interpreted as NO produced by direct photolysis but momentarily trapped at the surface. There is no evidence of any significant contributions from either substrate-mediated excitation or surface quenching.

ChemInform Abstract: Gas-Phase Enantioselectivity

Abstract Determination of the intrinsic noncovalent interactions governing chiral recognition in diastereomeric complexes constitutes the basis for understanding information transfer between molecules in living systems as well as in synthetic supramolecular structures. The most important experimental methodologies so far employed for this task are illustrated in the present review. Emphasis is put on the principles and the applications of techniques, such as radiolysis, Fourier transform ion cyclotron resonance (FTICR) and collision-induced dissociation (CID) mass spectrometry, and resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy, that allow measurement of the relative stability of diastereomeric ion/molecule and molecule/molecule complexes and quantification of the short-range forces controlling their enantioselective evolution to products.

Gas-phase enantioselectivity

Determination of the intrinsic noncovalent interactions governing chiral recognition in diastereomeric complexes constitutes the basis for understanding information transfer between molecules in living systems as well as in synthetic supramolecular structures. The most important experimental methodologies so far employed for this task are illustrated in the present review. Emphasis is put on the principles and the applications of techniques, such as radiolysis, Fourier transform ion cyclotron resonance (FTICR) and collision-induced dissociation (CID) mass spectrometry, and resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy, that allow measurement of the relative stability of diastereomeric ion/molecule and molecule/molecule complexes and quantification of the short-range forces controlling their enantioselective evolution to products.

Resonance enhanced multiphoton ionization time-of-flight study of CF2I2 photodissociation

The resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique was applied to study the complex dissociation of CF2I2 in a supersonic expansion. Using nanosecond excitation at 248, 266, and 304 nm and probing the I3/2 and/or I1/2 photoproducts, we determined the photofragment speed distribution, the recoil anisotropy parameter β and the branching ratio for each of the decay channels. At 248 nm the dissociation proceeds according to a concerted three-body decay, CF2I2→hvCF2+I1/2+I3/2 (90%) or CF2I2→hvCF2+I3/2+I3/2 (10%) consistent with our previous results obtained from photofragment translational spectroscopy. At 304 nm three competing decay channels were found to be operative. In decreasing order of branching, these are: two-body decay to CF2I +I1/2, sequential three-body decay CF2I2→ lim hvCF2I + I3/2→CF2 + I3/2+I3/2, and a concerted three-body decay yielding CF2 +I3/2+I3/2. The positive, almost maximum β values observed for all the primary steps in the 248, 266 and 304 nm photodissociation imply that the excited states which subsequently lead to dissociation are exclusively of B1 symmetry and that the primary bond-breaking processes occur on a subpicosecond time scale.

Infrared depletion spectroscopy of aniline dimer cation and aniline-benzene cation clusters

The vibrational spectra of the NH stretching modes of the aniline dimer cation and the aniline-benzene cluster cation have been measured by infrared depletion spectroscopy, which utilizes infrared multiphoton decomposition of the cluster ions and resonance-enhanced multiphoton ionization-time-of-flight (REMPI-TOF) mass spectrometry. Two strong absorption bands were observed at 3440 and 3305 cm −1 in the infrared spectrum of the aniline-benzene cluster cation, while the infrared spectrum of the aniline dimer cation exhibited only one strong peak at 3415 cm −1 in the region between 3200 and 3600 cm −1. A hydrogen bonded structure between one of the NH bonds of the aniline cation and the benzene ring for the aniline-benzene cluster ion, and that between the NH bond of the aniline cation and the lone pair of the nitrogen atom of neutral aniline for the dimer cation have been proposed.