Jhobe Steadman

Picosecond measurements of phenol excited‐state proton transfer in clusters. I. Solvent basicity and cluster size effects

Excited‐state proton transfer (ESPT) rates in molecular clusters were measured as a function of cluster size using picosecond spectroscopy in a molecular beam mass spectrometer. ESPT from the S1 state of phenol to base solvent clusters (NH3)n occurs for a critical solvent cluster size n≥5, with a rate constant of k=(60±10 ps)−1 for n=5–7. ESPT showing critical cluster‐size dependencies was also observed in the basic solvent N(CH3)3(n≂3). Proton transfer was not observed in the less‐basic solvent clusters (CH3OH)n and (H2O)n. Mixed‐solvent studies indicate that the addition of a dissimilar molecule to an otherwise neat solvent cluster impedes ESPT, presumably due to a disruption of the hydrogen bonding network. Evidence is also presented for the direct measure of solvent reorganization following ESPT. For (NH3)n solvation, the solvent reorganization appears as a long‐time‐scale component (0.3 ns) on the protonated solvent formation traces.

Picosecond mass‐selective measurements of phenol‐(NH3)n acid–base chemistry in clusters

The rate of proton transfer from the acidic S1 state of phenol to the basis solvent (NH3)n was measured as a function of solvent cluster size n. A distinct reaction threshold was observed for solvent size n=5 for 266 nm picosecond excitation. The proton transfer rate was measured to be ka=(60±10 ps)−1 for n=5–7. A competitive recombination rate of k−a =(350±100 ps)−1 occurs for n=5. Additional solvation stabilizes the product side causing the reaction enthalpy and consequently k−a to decrease. No evidence of proton transfer was observed when phenol was seeded in the less basic solvent clusters (CH3OH)n and (H2O)n.

Spectroscopy and dynamics of jet‐cooled hydrazines and ammonia. I. Single‐photon absorption and ionization spectra

Electronic‐state properties of hydrazine (N2H4), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single‐photon vacuum ultraviolet (vuv) absorption and photoionization spectroscopy in a molecular‐beam apparatus. The photoionization spectrum of NH3 shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C2 pyramidal gauche neutral geometry to a D2h planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N2H4), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuv absorption spectra of the hydrazines are also broad, which is to be contrasted with the well‐resolved spectrum of NH3. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular‐orbital model is developed to provide understanding...

A paraboloidal electrostatic reflector for molecular‐beam time‐of‐flight photoelectron spectrometers

We describe a design for a paraboloidal electrostatic reflector (PER) adapted for molecular‐beam time‐of‐flight (TOF) photoelectron spectrometers. The PER offers a nearly two orders of magnitude improvement in detection efficiency over standard line‐of‐sight TOF detection. The energy resolution ΔE/E is nominally about 0.02, but can be improved to about 0.005 (to a current limit of ΔE=10 meV) at some expense in sensitivity. The PER makes possible sensitive measurements for inherently weak ionization experiments. We have used our spectrometer in applications involving low‐power picosecond pulses, low‐density molecular clusters, and ionization through dissociative states.

Detection of neutral and ionic reaction mechanisms in molecular clusters

Techniques based on picosecond resonance-enhanced multiphoton ionization and mass-selective ion photodissociation are described as a means for studying complex reaction mechanisms in molecular clusters. A set of experiments is described that can identify the cluster-size specific reactions that occur by the overlapping processes of neutral dissociation/fragment ionization, parent ionization/dissociation, and parent ion photoexcitation/dissociation. These molecular beam techniques are demonstrated for the case of neutral and ionic photon transfer and evaporation in clusters of phenol in (NH(3))(n), as well for radical chemistry and van der Waals dissociation in (CH(3)I) (n) clusters.

Spectroscopy and photodissociation of size-selected (CH3I)n+ cluster ions

Abstract The resonant photodissociation of (CH 3 I) n + cluster ions ( n = 1–5) was studied in a crossed molecular-electron-laser beam apparatus. These results were complemented by a study of the ionization/dissociation as a function of variable energy electron ionization (EI). The fragmentation that first appears as the EI energy is raised is loss of CH 3 . This channel is particularly strong for n = 3. The lower energy dissociation of neutral I, on the other hand, is essentially quenched for cluster sizes of n > 2. Intermolecular dissociation (evaporation) of CH 3 I was evident, but was not extensive, based on a minor sensitivity of the cluster ion distribution to EI energy. The photodissociation of (CH 3 I) n + resulted in different dissociation behavior than for EI. Resonant excitation of monomer CH 3 I + to the predissociative A electronic state gave only I + CH 3 + . A state excitation of the cluster ions, on the other hand, resulted only in intermolecular dissociation. The photoexcitation cross-sections for the A 2 E 1/2 ← X 2 E 1/2 transition are presented as a function of cluster size n . A two-photon dissociation channel is also identified and the crosssections and dissociation products are reported. Finally, the frequency resolved photofragment excitation spectra of CH 3 I + and (CH 3 I) 2 + are compared.

A Vacuum Ultraviolet, Supersonic Jet Absorption Spectrometer

A technique for recording high-resolution, broadly tunable absorption spectra in the ultraviolet (UV) and vacuum ultraviolet (VUV) is described. A supersonic expansion is used to prepare molecules in a rovibrationally cold distribution of states, and a dispersed broad-band deuterium (D2) lamp is used for excitation. The attenuation of light due to absorption is measured with the use of dual gate detection during pulsed valve-on and valve-off periods. The VUV light beam, after crossing the supersonic expansion, is frequency down-converted with a scintillator-coated exit window, and the resulting light intensity is measured by boxcar integration or photon counting. A simple sample delivery system and complete software control of the experiment make this system a routine laboratory spectrometer. Sensitivity and resolution are discussed, and the device is demonstrated for the molecules NH3 and CH3I.

A time‐of‐flight mass filter for ion and cluster ion photodissociation studies

A sensitive method is described for detecting photodissociation products from a specific ion or cluster ion in the absence of the initially formed fragment ions that would otherwise interfere with the signals of interest. By using a simple pulse sequence, the conventional three‐grid time‐of‐flight (TOF) assembly can be operated as a low‐mass rejection filter capable of eliminating all ions below an adjustable threshold mass value. The method has been applied using different ionization sources [e.g., picosecond and nanosecond photoionization, and electron impact (EI) ionization]. Molecular‐beam applications are demonstrated for (1) high‐resolution resonance ion dissociation spectroscopy of rovibrationally cold ions, and (2) stepwise solvation of neat and seeded cluster ions formed by either photoionization or EI ionization. The low‐mass filter (LMF) is especially powerful for cluster ion photodissociation studies. For example, the elimination of successively larger cluster ions leads to photofragment mass ...

Improved Vacuum Ultraviolet Supersonic Jet Absorption Spectrometer

We report on design changes that have significantly improved the performance of our supersonic jet ultraviolet and vacuum ultraviolet absorption spectrometer. We incorporated the following three features: (1) direct detection of VUV photons rather than scintillator detection; (2) a triple-pass mirror arrangement; and (3) a larger-pathlength monochromator and a finer grating. These upgrades have yielded about an order-of-magnitude improvement in sensitivity and greater than fivefold improvement in resolution.

Structure-reactivity effects in the picosecond dynamics of isolated clusters

The link between gas-phase and condensed-phase chemical dynamics is being sought by the study of reactivity in small molecular clusters. We present measurements of reaction rates as a function of cluster size and composition to elucidate the role of solvent on a single-molecule basis. In previous work we showed that reactivity in clusters is strongly correlated to the stepwise binding energies of individual solvent molecules. Here, we show that solvent structure also plays a large role in determining chemical reactivity. This paper focuses on the excited state proton transfer reaction of phenol (PhOH) in a cluster of solvent-like molecules (i.e., NH3 and CH3OH). Three cases of structure-reactivity effects are reported: (1) rate inhibition by the addition of a CH3OH molecule to an (NH3) solvent cluster (60 ps vs 500 ps), (2) very different reaction rates for the inequivalent phenol molecules in phenol dimer solvated by (NH3) (50 ps vs 500 ps), and (3) solvent reorganization following proton transfer that occurs on the time scale of 0.3 ns.