Alexander Dalgarno. 5 January 1928—9 April 2015

Alexander (Alex) Dalgarno greatly advanced the quantitative study of fundamental atomic and molecular processes, contributed significantly to atmospheric science and ‘established molecular astrophysics as a unified intellectual field of great scientific endeavour, impact and achievement’ ( Flannery 2010 ). Alex developed and applied techniques that simplify calculations and lead to reliable solutions, enabling him to make landmark contributions to the knowledge of collisionally induced charge transfer, rotational and vibrational excitation of molecules, spin exchange and ultracold chemistry. His wide-ranging curiosity, disciplined steps to broaden his programme and ability to identify dominant physical processes and calculate their rates led to his many important contributions to atmospheric science. For example, Alex greatly expanded the knowledge of terrestrial airglow features, photoabsorption and collisional processes in the terrestrial ozone layer and deposition by energetic electrons in the atmospheres of other planets. In molecular astrophysics, he applied that same systematic approach to studies of a range of environments from the early Universe to present-day UV-irradiated interstellar clouds, shocks and supernova ejecta. Alex made availability to students a priority and encouraged them to pursue problems that they devised, despite his seemingly inexhaustible supply of suitable projects. His community service included his 29-year long editorship of Astrophysical Journal Letters , starting in 1973, and the founding of the Institute of Theoretical Atomic and Molecular Physics at the Harvard-Smithsonian Center for Astrophysics, in 1988, owing to his concerns for the health of the fields of theoretical atomic and molecular physics and fundamental quantum mechanics, which the Institute did much to reinvigorate.

The smallest proton-bound dimer H 5 + : theoretical progress

The protonated hydrogen dimer, H 5 + , is the smallest system including proton transfer, and has been of long-standing interest since its first laboratory observation in 1962. H 5 + and its isotopologues are the intermediate complexes in deuterium fractionation reactions, and are of central importance in molecular astrophysics. The recently recorded infrared spectra of both H 5 + and D 5 + reveal a rich vibrational dynamics of the cations, which presents a challenge for standard theoretical approaches. Although H 5 + is a four-electron ion, which makes highly accurate electronic structure calculations tractable, the construction of ab initio -based potential energy and dipole moment surfaces has proved a hard task. In the same vein, the difficulties in treating the nuclear motion could also become cumbersome due to their high dimensionality, floppiness and/or symmetry. These systems are prototypical examples for studying large-amplitude motions, as they are highly delocalized, interconverting between equivalent minima through internal rotation and proton transfer motions requiring state-of-the-art treatments. Recent advances in the computational vibrational spectroscopy of the H 5 + cation and its isotopologues are reported from full quantum spectral simulations, providing important information in a rigorous manner, and open perspectives for further future investigations. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H 3 + , H 5 + and beyond’.

PAH under XUV excitation: an ultrafast XUV-photochemistry experiment for astrophysics

Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

Theory, experiment, and simulations in laboratory astrochemistry

This Special Issue of PCCP acknowledges the synergies between laboratory and observational molecular astrophysics. It forms a collection of experimental as well as theoretical physical-chemistry investigations with the understanding of astrochemical processes as common background.

The molecular universe: from observations to laboratory and back

This brief overview stresses the importance of molecular processes in modern astrophysics and provides examples where the availability of new laboratory or theoretical data proved crucial in the analysis. This includes basic data such as spectroscopy and collisional rate coefficients, but also an improved understanding of reactions and photoprocesses in the gaseous and solid state. In spite of many lingering uncertainties, the future of molecular astrophysics is bright with new facilities such as ALMA, JWST and ELTs on the horizon. Together, they will allow increased understanding of the journey of gas and solids from clouds to stars and planets, and back to the interstellar medium.

New Scientific Facilities and Directions in Molecular Astrophysics

A brief overview is given of major current and future observational facilities which will enable new scientific opportunities in the study of interstellar and circumstellar molecules , ranging from the early universe to protoplanetary disks and exoplanatory atmospheres. Close interaction between astronomers and laboratory astrophysicists will be essential to read the full scientific harvest from these major investments.The Himalayan Physics Vol. 4, No. 4, 2013 Page: 102-105 Uploaded date: 12/23/2013