Sridhar A. Lahankar

Megapixel ion imaging with standard video

We present an ion imaging approach employing a real-time ion counting method with standard video. This method employs a center-of-mass calculation of each ion spot (more than 3×3pixels spread) prior to integration. The results of this algorithm are subpixel precision position data of the corresponding ion spots. These addresses are then converted to the final image with user selected resolution, which can be up to ten times higher than the standard video camera resolution (640×480). This method removes the limiting factor imposed by the resolution of standard video cameras and does so at very low cost. The technique is used in conjunction with dc slice imaging, replacing the local maximum searching algorithm developed by Houston and co-workers [B. Y. Chang, R. C. Hoetzlein, J. A. Mueller, J. D. Geiser, and P. L. Houston, Rev. Sci. Instrum. 69, 1665 (1998)]. The performance is demonstrated using HBr and DBr photodissociation at 193nm with 3+1 resonance enhanced multiphoton ionization detection of hydrogen ...

Electronic interactions in a branched chromophore investigated by nonlinear optical and time-resolved spectroscopy

The third order nonlinear optical properties of a trimer branched chromophore system and its linear molecule analog are investigated. Two-photon absorption and degenerate four wave mixing measurements were carried out on both systems. An enhancement in the nonlinear optical effect is observed for the branched trimer molecule in comparison to the linear chromophore system. Ultrafast time-resolved measurements were carried out to probe the excited state dynamics in the branched structures. The time-resolved measurements suggest that the two important processes affecting the nonlinear optical properties in the trimer system, charge transfer stabilization and initial electronic delocalization, occur on two different time scales.

Pyrolysis of Phenolic Impregnated Carbon Ablator (PICA)

Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H2O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH4, CO2, and phenol and its methylated derivatives; these products tended to desorb concurrently with H2O and CO, over the range from about 200 to 600 °C. H2 is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H2 was not quantified. The observation of a much higher yield of CO than CH4 suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH4 in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.

State-correlated DC slice imaging of formaldehyde photodissociation: roaming atoms and multichannel branching

High-resolution slice imaging methods allow for detection of single product quantum states with sufficient velocity resolution to infer the full correlated product state distribution of the undetected fragment. This is a level of detail not available in previous studies of formaldehyde photodissociation, and in this application it reveals startling new aspects of unimolecular decomposition. The CO rotational distributions from near ultraviolet dissociation of formaldehyde are bimodal, and the imaging experiments allow us to decompose these into two dynamically distinct components: the conventional molecular dissociation over a high exit barrier, and a novel ‘roaming atom’ reaction in which frustrated radical dissociation events lead to intramolecular H abstraction, bypassing the transition state entirely. In probing the dynamics of the conventional molecular dissociation over the barrier, we use the complete vH2-jCO correlation to model the exit channel dynamics in new detail. Furthermore, these state-correlated measurements provide insight into radical–radical reactions and the underlying dynamics and energy dependence of the roaming pathway.

Correlated vH2 and jCO product states from formaldehyde photodissociation: Dynamics of molecular elimination

A detailed study of the photoinduced molecular elimination pathway of formaldehyde on the ground state surface was carried out using high-resolution dc slice ion imaging. Detailed correlated H(2) rovibrational and CO rotational product quantum state distributions were measured by imaging spectroscopically selected CO velocity distributions following photodissociation at energies from approximately 1800 to approximately 4100 cm(-1) above the barrier to molecular elimination. Excitation to the 2(1)4(1), 2(1)4(3), 2(2)4(1), 2(2)4(3), and 2(3)4(1) bands of H(2)CO are reported here. The dependence of the product rovibrational distributions on excitation energy are discussed in light of a dynamical model which has been formulated to describe the strong product state correlations observed.

Crossed-beams studies of the dynamics of the H-atom abstraction reaction, O(3P) + CH4 → OH + CH3, at hyperthermal collision energies.

The H-atom abstraction reaction, O((3)P) + CH(4) → OH + CH(3), has been studied at a hyperthermal collision energy of 64 kcal mol(-1) by two crossed-molecular-beams techniques. The OH products were detected with a rotatable mass spectrometer employing electron-impact ionization, and the CH(3) products were detected with the combination of resonance-enhanced multiphoton ionization (REMPI) and time-sliced ion velocity-map imaging. The OH products are mainly formed through a stripping mechanism, in which the reagent O atom approaches the CH(4) molecule at large impact parameters and the OH product is scattered in the forward direction: roughly the same direction as the reagent O atoms. Most of the available energy is partitioned into product translation. The dominance of the stripping mechanism is a unique feature of such H-atom abstraction reactions at hyperthermal collision energies. In the hyperthermal reaction of O((3)P) with CH(4), the H-atom abstraction reaction pathway accounts for 70% of the reactive collisions, while the H-atom elimination pathway to produce OCH(3) + H accounts for the other 30%.

Novel molecular elimination mechanism in formaldehyde photodissociation: the roaming H atom pathway

We present a state-correlated experimental investigation of formaldehyde (H2CO) dissociation to H2 and CO following excitation to a series of vibrational bands in the first electronically excited state, S1. The CO was detected by resonance-enhanced multiphoton ionization at various rotational states of CO (J = 5–45) and the CO velocity distributions were measured using state-resolved DC Slice Imaging. These high-resolution measurements reveal the internal state distribution of the correlated H2 cofragments. The results show that the rotationally hot CO (JCO = 40) is produced in conjunction with vibrationally cold H2 fragments (ν = 0–3), consistent with dissociation through the celebrated skewed transition state. After excitation of formaldehyde at energies near and above the threshold for dissociation to radical products (H2CO → H+HCO), a second molecular elimination channel appears which is characterized by rotationally cold CO (J 5–15) correlated with highly vibrationally excited H2 (ν = 5–7). These products are formed through a novel roaming H-atom mechanism that involves intramolecular H abstraction and avoids the region of the transition state to molecular elimination entirely. The current measurements give insight into the energy dependence of the branching of these different reaction mechanisms.

Doppler-free/Doppler-sliced ion imaging

We demonstrate a hybrid Doppler-free/Doppler-sliced ion imaging approach that is well-suited for detection of H or D atoms. The method relies on 2 + 1 resonant ionization with identical, nearly counterpropagating beams that are coplanar but directed at a small angle relative to the detector face. This results in Doppler selection of the velocity component along the time of flight axis but Doppler-free detection in the plane perpendicular to this axis. The results show high signal level and excellent slicing ( approximately 5%), yielding velocity resolution completely determined by cation recoil in the ionization step.

Electronic Population Inversion in HCCO/DCCO Products from Hyperthermal Collisions of O(3P) with HCCH/DCCD

The dynamics of hyperthermal O((3)P) reactions with acetylene have been investigated with the use of crossed molecular beams techniques, employing both mass spectrometric and optical detection of products. With collision energies of 40-150 kcal mol(-1), O((3)P) + HCCH/DCCD → HCCO/DCCO + H/D may follow multiple pathways to form the ketenyl radical (HCCO or DCCO) in ground doublet states or in electronically excited quartet and doublet states. Theoretical calculations support the assignment of the various reaction pathways. The fraction of electronic excitation is substantial. At the highest collision energy studied, ∼65% of the ketenyl radical products that survive are electronically excited, with the majority of the excited products in a quartet state. In this case, a population inversion exists between the electronically excited quartet and ground doublet states of the ketenyl product. Such significant electronic excitation in products is unusual in bimolecular reactions, especially when ground-state products are accessible by spin-allowed pathways.