Paul L. Houston

Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization

A new technique is presented that makes it possible, with a single laser pulse, to determine the three‐dimensional spatial distribution of state‐selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three‐dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state‐selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital‐image processing device such as a two‐dimensional optical multichannel analyzer. The three‐dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments...

Solid-state electroluminescent devices based on transition metal complexes

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.

Methyl rotation, vibration, and alignment from a multiphoton ionization study of the 266 nm photodissociation of methyl iodide

The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame...

Improved two-dimensional product imaging: The real-time ion-counting method

A novel ion-counting method for significantly improving the spatial resolution and detection sensitivity of two-dimensional product imaging in molecular beam experiments is presented. The method makes use of real-time digital image processing to retrieve, threshold, and determine the local maximum of each ion hitting a microchannel plate assembly. The current version can process data at rates up to 3.07 Mbyte/s, and methods for accelerating this rate are proposed.

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Product state distributions of the CO produced in the 308-nm photolysis of acetaldehyde show clear evidence of two dissociation mechanisms. One is attributed to the conventional transition state mechanism predicted by theory, with high rotational and translational energy of the CO and a pronounced v⊥J vector correlation. However, as much as 15% of the reaction flux proceeds via another pathway that produces low CO rotational and translational energy, very high CH4 internal energy, and no correlation between the CO velocity and angular momentum vectors. The attributes of this channel are dynamically similar to the recently reported “roaming atom” mechanism in formaldehyde. We therefore speculate that the second pathway in acetaldehyde also occurs via a roaming mechanism in the CH3 + HCO exit channel that decays into the CH4 + CO channel.

FORMALDEHYDE PHOTOCHEMISTRY: APPEARANCE RATE, VIBRATIONAL RELAXATION, and ENERGY DISTRIBUTION OF THE CO PRODUCT

The mechanism of formaldehyde photochemistry has been investigated by monitoring the appearance rate, relative yield, and vibrational distribution of the CO photochemical product detected either by its infrared fluorescence or by its absorption of a cw CO laser. In the limit of low formaldehyde pressures, the CO product appears with a rate more than 100 times slower than the decay rate of the formaldehyde first excited singlet state. This fact indicates the presence of a long‐lived intermediate state between S1 and the molecular products. Collision‐induced CO production following 337.1 nm formaldehyde excitation occurs with appearance rates of 2.7×10−11 cm3 molecule−1⋅sec−1 for D2CO and 4.7×10−11 cm3 molecule−1⋅sec−1 for H2CO. After its production, CO(v=1) relaxes to the ground vibrational state in collisions with D2CO at a rate of 3.3×10−12 cm3 molecule−1⋅sec−1 and in collisions with H2CO at a rate of 3.7×10−14 cm3 molecule−1⋅sec−1. These rates have been confirmed by a separate measurement which monitors...

Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions

The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum‐ultraviolet laser‐induced fluorescence was used to probe the CO fragment, and multiphoton ionization time‐of‐flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.

The temperature dependence of absolute rate constants for the F+H2 and F+D2 reactions

Multiphoton dissociation of SF6 has been used to generate a transient concentration of fluorine atoms in mixtures with argon and H2 or D2. Absolute rate constants for the F+H2 and F+D2 reactions have been determined as a function of temperature by monitoring the appearance rate of HF or DF product chemiluminescence. In the temperature range from 190 °K to 373 °K the results are fit by the expressions kH=1.0×10−10 exp(−(860±100)/RT) and kD=9.1×10−11 exp(−(1100±100)/RT), both in cm3 molecule−1 sec−1. These values are in rough agreement with those obtained recently using a similar technique. The value of the isotope effect kH/kD is in good agreement with two previously determined values.

Differential cross sections for state‐selected products by direct imaging: Ar+NO

State‐selected differential cross sections have been obtained by directly imaging the products of collisions in crossed molecular beams. The new technique allows final state resolution and simultaneous detection of all scattering angles. The method has been used to study inelastic collisions between Ar and NO(2Π1/2, υ=0, J=0.5) at a collision energy of 0.21 eV. Rotational rainbows in the product angular distribution are directly observed to change in position as a function of the final rotational state; the peak of the angular distribution moves toward the backward hemisphere and the angular distribution broadens with an increase in final rotational quantum number. The method relies on multiphoton ionization of the product but is otherwise generally applicable to reactive as well as inelastic collisions.

State‐resolved photodissociation of OCS monomers and clusters

Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four‐wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one...