Sanghamitra Deb

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Ultrafast structural dynamics in Rydberg excited N,N,N′,N′-tetramethylethylenediamine: conformation dependent electron lone pair interaction and charge delocalization

Two nitrogen atoms and a flexible carbon skeleton make N,N,N′,N′-tetramethylethylenediamine (TMEDA) an important model system to study the interplay of conformeric motions and charge delocalization. Ionization of one of the nitrogen atoms generates a localized charge that may (partially) transfer to the other nitrogen. The structural motions, conformation dependent electron lone pair interaction and charge transfer in Rydberg-excited TMEDA, where the molecular ion core closely resembles the ion, were probed by time-resolved Rydberg fingerprint spectroscopy. Excitation to the 3p Rydberg level with a 209 nm laser pulse initially created a charge-localized ion core. Rapid internal conversion to the 3s Rydberg state yielded a multitude of conformational structures, in particular structures that are close to the folded GG′G+ and GGG′+ (see text for label definitions) core structures (235 fs), and structures that are close to the extended TTT+ core structure (557 fs). The initial excitation and the internal conversion deposit about 1.89 eV of energy into the vibrational manifold, enabling a fast equilibrium between the folded and the extended structures. The forward and backward time constants were determined to be 490 fs and 621 fs, respectively. With the molecule highly vibrationally excited, the decay to 3s proceeds with a 6.77 ps time constant. Density functional theory (DFT) and ab initio calculations show evidence of strong lone pair interaction and charge delocalization in the equilibrium conformers. Importantly, DFT with self-interaction correction properly describes the binding energy of the Rydberg electron and provides excellent agreement with the experimental results.

Structural Dynamics in Floppy Systems: Ultrafast Conformeric Motions in Rydberg-Excited Triethylamine

Rotations about its three carbon-nitrogen bonds give triethylamine a complex, 3-dimensional potential energy landscape of conformeric structures. Electronic excitation to Rydberg states prepares the molecule in a high-energy, nonequilibrium distribution of such conformers, initiating ultrafast transitions between them. Time-resolved Rydberg electron binding energy spectra, observed using photoionization-photoelectron spectroscopy with ultrashort laser pulses, reveal these time-evolving structures. The time-dependent structural fingerprint spectra are assigned with the aid of a computational analysis of the potential energy landscape. Upon 209 nm electronic excitation to the 3p Rydberg state, triethylamine decays to 3s with a 200 fs time constant. The initially prepared conformer reacts to a mixture of structures with a time constant of 232 fs and settles into a final geometry distribution on a further subpicosecond time scale. The binding energy of the Rydberg electron is found to be an important determinant of the conformeric energy landscape.

Probing the lifetimes of internally excited amyl nitrite cations.

The photoelectron spectrum shows that multiphoton ionization of amyl nitrite, C(5)H(11)ONO, using ultrafast laser pulses deposits up to 3.7 eV of energy into internal degrees of freedom. As a result, the molecules fragment to produce various daughter ions of masses 87, 71, 60, 57, 41, 30, 29, and 27. Absorption of an additional photon with 3 eV of energy by the ions yields transients with picosecond decay times, revealing the time scale of the decomposition dynamics of the initially prepared parent ion. Each mass peak has a distinct time constant, in the range of 1.2 to 7.9 ps, emphasizing the dependence of the fragmentation mechanism on the ion internal energy.

Observation of Structural Wavepacket Motion: The Umbrella Mode in Rydberg-Excited N-Methyl Morpholine

We have observed time-resolved, structural dynamics of a coherent vibrational wavepacket in Rydberg-excited N-methyl morpholine, a molecule with 48 internal degrees of freedom. The molecular structure was established by associating the time-dependent Rydberg electron binding energy, obtained from time-resolved photoionization-photoelectron spectroscopy, to the molecular structure using self-interaction corrected density functional calculations. Optical excitation at 226 nm launches an oscillatory wavepacket in the amine umbrella coordinate with a 650 fs period. Even though the Franck-Condon excitation is at an angle of 17°, the wavepacket settles into an oscillation between 4° and -10° within a fraction of a vibrational period and then dephases with a time constant of 750 fs.

In vivo pump-probe microscopy of melanoma: characterizing shifts in excited state photodynamics with respect to invasiveness

Pump-probe microscopy is a multiphoton technique that generates molecular contrast from absorptive pigments, such as melanin. It holds the potential to be used as a non-invasive screening tool to discern whether a given early-stage melanoma has acquired the capacity for metastasis. Here, we examined lesions in a Braf(V600E)-driven model of melanoma to assess whether loss of the tumor suppressor gene Pten in a is accompanied by a shift in pigment expression, as measured in vivo by pump-probe microscopy. The data were analyzed to determine differences in the excited-state lifetime of melanins expressed in Pten-competent and Pten-loss pigmented lesions. Loss of the tumor suppressor Pten was found to be accompanied by a statistically significant decrease in pixel-average excited state lifetime (p = 1.3e-4).

Far-UV photochemical bond cleavage of n-amyl nitrite: bypassing a repulsive surface.

We have investigated the deep-UV photoinduced, homolytic bond cleavage of amyl nitrite to form NO and pentoxy radicals. One-color multiphoton ionization with ultrashort laser pulses through the S(2) state resonance gives rise to photoelectron spectra that reflect ionization from the S(1) state. Time-resolved pump-probe photoionization measurements show that upon excitation at 207 nm, the generation of NO in the v = 2 state is delayed, with a rise time of 283 (16) fs. The time-resolved mass spectrum shows the NO to be expelled with a kinetic energy of 1.0 eV, which is consistent with dissociation on the S(1) state potential energy surface. Combined, these observations show that the first step of the dissociation reaction involves an internal conversion from the S(2) to the S(1) state, which is followed by the ejection of the NO radical on the predissociative S(1) state potential energy surface.

Comparison of pump-probe and hyperspectral imaging in unstained histology sections of pigmented lesions

Microscopic variations in melanin composition can be mapped through linear and nonlinear optical responses. Though instrumentation to measure linear attenuation is simple and inexpensive, the nonlinear response provides more degrees of freedom with which to spectroscopically resolve pigments. The objective of this study is to assess differences in imaging melanin contrast by comparing hyperspectral (linear) versus pump-probe (nonlinear) microscopy of unstained histology sections of pigmented lesions. The images and analysis we have presented here show that pump-probe uncovers a greater variation in pigment composition, compared with hyperspectral microscopy, and that the two methods yield complimentary biochemical information.

Dual-wavelength pump-probe microscopy analysis of melanin composition

Pump-probe microscopy is an emerging technique that provides detailed chemical information of absorbers with sub-micrometer spatial resolution. Recent work has shown that the pump-probe signals from melanin in human skin cancers correlate well with clinical concern, but it has been difficult to infer the molecular origins of these differences. Here we develop a mathematical framework to describe the pump-probe dynamics of melanin in human pigmented tissue samples, which treats the ensemble of individual chromophores that make up melanin as Gaussian absorbers with bandwidth related via Frenkel excitons. Thus, observed signals result from an interplay between the spectral bandwidths of the individual underlying chromophores and spectral proximity of the pump and probe wavelengths. The model is tested using a dual-wavelength pump-probe approach and a novel signal processing method based on gnomonic projections. Results show signals can be described by a single linear transition path with different rates of progress for different individual pump-probe wavelength pairs. Moreover, the combined dual-wavelength data shows a nonlinear transition that supports our mathematical framework and the excitonic model to describe the optical properties of melanin. The novel gnomonic projection analysis can also be an attractive generic tool for analyzing mixing paths in biomolecular and analytical chemistry.

Enhancing Pigmented or Transparent Tissue Imaging with Laser Pulse Shaping

Enhanced control over femtosecond lasers (pulse shaping or pulse train modulation) improves contrast in tissue imaging. Pump-probe applications to melanoma diagnosis and cross phase modulation measurement in transparent tissues will be presented.