T.-H. Wong

Photodissociation spectroscopy of MgCH+4

The photodissociation spectroscopy of MgCH+4 has been studied in a reflectron time‐of‐flight mass spectrometer. MgCH+4 molecular absorption bands are observed to the red of the Mg+(3 2PJ←3 2S1/2) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive (Mg+), and two reactive fragmentation products (MgH+ and MgCH+3) are observed, all with similar action spectra. The product branching is independent of wavelength, Mg+:MgCH+3:MgH+∼60:33:7. The absorption is assigned to the transition (1 2E←1 2A1) in C3v symmetry (with η3 coordination), followed by a geometrical relaxation of the complex toward states of 2B1 and 2B2 symmetry in C2v geometry (with η2 coordination). Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg+ product angular distri...

Photodissociation spectroscopy and dynamics of MgC2H4

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fa...

PHOTOFRAGMENTATION SPECTROSCOPY OF AL+(C2H4)

We have studied the structure and dissociation dynamics of the weakly bound bimolecular complex Al+(C2H4) by photodissociation spectroscopy in the 216–320 nm spectral region. Experimental studies are supported by ab initio electronic structure calculations of the ground and low-lying excited states of the complex. Al+ is the dominant photofragment observed throughout the absorption profile. C2H4+ charge transfer product is also observed for shorter photolysis wavelengths, λ<252 nm. The Al+–C2H4 bond dissociation energy is measured as D0=0.37±0.15 eV. Three molecular absorption bands are observed and assigned to the transitions (2 1A1,1 1B1,1 1B2←1 1A1) in C2v equilibrium complex geometry. The excited states are of predominantly charge-transfer character correlating with the product channel Al(3s23p)+(C2H4)+. The 2 1A1 and 1 1B2←1 1A1 absorption bands appear broad and structureless. This observation is consistent with ab initio results that suggest a pathway for rapid nonadiabatic dissociation through a 1 ...

Electronic orbital alignment effects in the reaction Mg*(3p 1P1)+CH4→MgH+CH3

We have measured the final state resolved far‐wing action spectra for the MgCH4 reactive collision system. The results show a dramatic ‘‘Π‐like’’ orbital alignment preference in the reaction channel. The reactive channel action spectra for different MgH rotational states in v=0 are identical, suggesting that the reaction follows from a single approach geometry, with the product rotational distribution determined by exit channel effects. Based on these observations and molecular orbital considerations, we propose that the reaction proceeds in η2 approach geometry through a triangular C–Mg–H transition state.

Chemical dynamics of the reaction K*(5p 2P)+H2→KH(v=0;J)+H: Electronic orbital alignment effects

We report results from scattering state spectroscopic studies of the excited state reaction K*(5p 2P)+H2→KH(v″,J″)+H. The final state resolved action spectra allow a direct measurement of essential features of the excited state potential surfaces, including regions of local maxima and minima. We observe a pronounced blue-wing–red-wing asymmetry in the reactive to nonreactive branching ratio, peaking in the neighborhood of a strong blue wing satellite. These results show that the dominant reaction pathway passes over a small activation barrier (350±100 cm−1) in Σ+-like orbital alignment. This result is consistent with an electron jump mechanism through a K+H−H ion-pair intermediate. In contrast, approach in Π-like alignment leads predominantly to nonreactive scattering. Our results suggest that a combination of steric and energetic effects determine the major quenching pathways for alkali metal atom-H2 systems.

Scattering state spectroscopy of the reaction Mg*(3s3p 1P1)+CH4→MgH(v=0,1;N)+CH3

We report scattering state spectroscopic studies of the chemical quenching dynamics of Mg*(3p(1P)) by CH4. We have measured the final-state resolved action spectra for the MgH(v=1,N) reactive product channels, following excitation of the Mg*(3p)–CH4 transient bimolecular collision complex. As in earlier work on the ground vibrational state of the product, we have found a strong electronic orbital alignment effect: Reaction to the vibrationally excited product follows only on the attractive excited potential-energy surfaces in “Π-like” symmetry. For both MgH(v=0 and 1) product channels we have found that the rotational quantum state distribution is independent of laser excitation wavelength, indicating that the rotational energy partitioning is determined by exit channel dynamics. However, our results show that the product vibrational energy disposal is a function of excitation laser wavelength, suggesting that the vibrational energy partitioning is correlated with the collisional impact parameter. We have...

Photodissociation spectroscopy and dynamics of Mg+-formaldehyde

We have carried out photodissociation spectroscopy studies of the bimolecular complex Mg+(H2CO) in the visible and near-uv regions. The work is supported by electronic structure calculations of the ground and low-lying excited states of the complex. Mg+-formaldehyde is bound in a C2v Mg+–O=CH2 geometry with a theoretical bond energy of De″(Mg-OCH2)=1.35 eV. The complex shows absorption bands that correlate with Mg+-based and formaldehyde-based radiative transitions. The lowest-energy band is assigned as A 2A′(2B1)←X 2A1, to an excited state of mixed Mg+(3pπ) and H2CO(π*) orbital character. The band exhibits complex vibrational structure with considerable excitation of the CH2 out-of-plane wag and C=O stretch modes; the vibrational frequencies are shifted dramatically from their values in the ground state, showing the effect of a significant weakening of the C=O bond and out-of-plane distortion of the complex. Excitation in the Mg+-based B 2A′(2B2)←X 2A1 band shows predominantly low-frequency vibrationa...

Photochemistry of Zn+(CH4)

Abstract We report on the photodissociation spectroscopy of Zn+(CH4). Two overlapping continuum absorption bands are observed in the spectral range 227–263 nm. The bands are assigned to transitions correlating with Zn+-based (4p←4s) excitation and photoinduced charge transfer, respectively. The reactive product ZnCH3+ is the major dissociation product showing that the dominant quenching pathway involves C–H bond insertion. Additional dissociation products include CH3+,Zn+, and ZnH+ with a relative branching that depends on excitation energy. Results are compared to previous studies of the chemical quenching of light metal atoms and ions by CH4.

Collisional energy transfer in Na(4p–3d)–He,H2 collisions

We have investigated the direct collisional energy transfer process Na*(4p)+M→Na*(3d)+M, where M=He,H2 under gas cell conditions. We have measured the temporal profiles of the Na(3d–3p) sensitized fluorescence as a function of quenching gas pressure and fit the profiles to a two‐state rate equation model to obtain the quenching rate coefficients from the Na*(4p) state. The total energy transfer rate coefficient out of the 4p state for He is small [(0.5±0.2)×10−10 cm3/s]. The total quenching rate coefficient out of the 4p state is much larger for H2[(3.9±0.5)×10−10 cm3/s]. Evidence suggests that the energy transfer rate coefficient for the 4p–3d process is ∼2.0×10−10 cm3/s with the remainder of the 4p quenching being predominantly reactive. We also compare the far‐red wing absorption line shapes for the NaHe and NaH2 systems.

Photodissociation spectroscopy of Ca+-formaldehyde

We have carried out photodissociation spectroscopy studies of Ca+(H2CO). The work is supported by electronic structure calculations of the ground and low-lying excited states of the complex. Our results show that Ca+(H2CO) is bound in a C2v Ca–O–CH2 ground-state equilibrium geometry with a bond dissociation energy of De″(Ca–OCH2)=0.9±0.2 eV. We have assigned five distinct molecular absorption bands of the complex that correlate with Ca+-based 3d and 4p←4s radiative transitions. The absorption bands show clear vibrational resonance structure with partially resolved rotational substructure. Spectroscopic constants are, for the most part, in good agreement with ab initio model predictions. The experimental data give quantitative information about the bonding interactions for electronically excited states of different valence and molecular-orbital character. Results for Ca+(H2CO) are compared with previous results from our lab on the analogous Mg+(H2CO) complex [J. Chem. Phys. 117, 6970 (2002)].