Website Building SoftwareVenue: AB2-103; Time: 15:30 hrs
Speaker:
Dr. Arnab Sen
Max-Born Institute, Berlin
Understanding photochemical processes in molecules remains a fundamental challenge, as they involve coupled electronic–nuclear dynamics that often extend beyond the validity of the Born–Oppenheimer approximation (BOA). In such situations, molecular potential energy surfaces can become degenerate, particularly in the vicinity of conical intersections (CIs). These intersections act as ultrafast funnels for radiationless transitions and play a central role in numerous photochemical processes in nature. Notable examples include the cis–trans isomerization of the retinal chromophore in rhodopsin, the remarkable photostability of UV-excited DNA and RNA nucleobases, and the ring-opening reaction leading to the formation of pre-vitamin D₃. Real-time observation of coupled electronic–nuclear dynamics is therefore essential for achieving predictive control over photochemical reactivity in areas such as biological systems, atmospheric chemistry, photocatalysis, and solar energy conversion. The earliest stages of photoinduced processes occur on ultrashort timescales, ranging from a few to several hundred femtoseconds, and involve vibronically coupled electronic–nuclear wavepackets. Experimentally capturing these dynamics remains highly challenging, as it requires both ultrafast temporal resolution and sensitivity to electronic coherence and nuclear rearrangement.
Advances in ultrafast laser technology and pump–probe spectroscopy—particularly following the pioneering work of Ahmed Zewail, later recognized with the Nobel Prize in Chemistry—have enabled direct observation of molecular dynamics on their intrinsic femtosecond timescales. While many time-resolved techniques primarily probe either nuclear motion (e.g., vibrational spectroscopy and diffraction) or electronic dynamics (e.g., attosecond streaking), most photochemical processes are governed by strongly coupled electronic and nuclear motion. Capturing this interplay therefore requires techniques that are simultaneously sensitive to both degrees of freedom. In this context, time-resolved photoelectron spectroscopy (TRPES) and time-resolved X-ray absorption spectroscopy (TRXAS) have emerged as powerful tools for probing coupled electronic–nuclear wavepacket dynamics.
In this talk, the speaker will focus on TRPES, in which tuneable, few-femtosecond vacuum-ultraviolet (VUV) pulses generated via resonant dispersive wave (RDW) emission in gas-filled hollow-core fibers are used to probe the early-time dynamics of photoexcited ethylene and its deuterated isotopologue. This approach enables the direct observation of a previously unrecognized mechanism governing the initial stages of ethylene’s ultrafast excited-state dynamics. In particular, a strong nonadiabatic coupling between the initially populated π π* state and the σ π* state drives rapid torsional motion in VUV-excited ethylene, resulting in substantial population transfer between these states within less than 10 fs. Also, the speaker will present their recent work using TRXAS, in which ultrashort X-ray pulses generated from a table-top high-harmonic-generation source are employed to investigate the ultrafast structural rearrangement of nitromethane following strong-field ionization. These measurements are the first of their kind, to identify the transient intermediate involved in the nitro–nitrite rearrangement.
Past Seminars
Speaker:
Dr. Sai Chaitanya
Indiana University, USA
Our current best-fit cosmological model explains structure formation primarily through adiabatic fluctuations. Complementary and orthogonal to these are isocurvature fluctuations. Primordial isocurvature is undetected on CMB scales and often treated as a constrained nuisance within LambdaCDM, but it can also act as a structured degree of freedom with real impact on cosmological inference. In this talk I will present an isocurvature-forward program: we study CDM/axion isocurvature sourced by a new "hyperbolic geometry" mechanism with scale-dependent (often blue-tilted) spectra, construct data-ready templates, and quantify their impact within standard analyses. I will highlight an application where a subdominant CDM component carrying isocurvature compensates the power suppression from dominant warm dark matter, reopening parameter space and allowing WDM masses as low as 300 eV (about an order of magnitude below typical current WDM mass bounds). I will briefly discuss extensions to ultra-light axion (fuzzy) dark matter, and conclude with comments on unexplored degeneracies between isocurvature contributions and neutrino-mass inference.
Speaker:
Prof. Aseem Paranjape
IUCAA, Pune
The baryon acoustic oscillation (BAO) feature in the clustering of biased tracers of cosmological matter, such as galaxies, is a key prediction of the Big Bang model of the evolution of the Universe, rooted in well-understood atomic physics. Being a standard ruler, the BAO feature plays a critical role in cosmological inference using large-volume galaxy redshift surveys. Traditional BAO analyses typically build on templates derived from assuming a cosmological model such as standard Lambda-cold dark matter (LCDM). In order to convincingly test LCDM (or any model), however, one requires a model-agnostic description of the BAO feature involving a number of ingredients. Physically, one must describe the impact of cosmological bulk flows which progressively and anisotropically smear out the feature over time. One must also model nonlinear effects such as the scale dependence of tracer bias and the gravitational coupling between short and long scales. All of these can be incorporated using the so-called Zel'dovich approximation, without reference to any particular cosmological model. On the technical front, one needs a robust, complete and cosmology-independent basis of 1-dimensional functions to describe the shape of the BAO feature in linear theory, which can then be propagated to the nonlinearly evolved, measured feature. I will describe how these ingredients -- which have been systematically constructed in recent work -- come together in an accurate framework capable of describing the BAO-scale pairwise measurements of state-of-the-art galaxy surveys, thus enabling model-agnostic cosmological inference.
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