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.

Atomic and Molecular Processes in a Strong Bicircular Laser Field

With the development of intense femtosecond laser sources it has become possible to study atomic and molecular processes on their own subfemtosecond time scale. Table-top setups are available that generate intense coherent radiation in the extreme ultraviolet and soft-X-ray regime which have various applications in strong-field physics and attoscience. More recently, the emphasis is moving from the generation of linearly polarized pulses using a linearly polarized driving field to the generation of more complicated elliptically polarized polychromatic ultrashort pulses. The transverse electromagnetic field oscillates in a plane perpendicular to its propagation direction. Therefore, the two dimensions of field polarization plane are available for manipulation and tailoring of these ultrashort pulses. We present a field that allows such a tailoring, the so-called bicircular field. This field is the superposition of two circularly polarized fields with different frequencies that rotate in the same plane in opposite directions. We present results for two processes in a bicircular field: High-order harmonic generation and above-threshold ionization. For a wide range of laser field intensities, we compare high-order harmonic spectra generated by bicircular fields with the spectra generated by a linearly polarized laser field. We also investigate a possibility of introducing spin into attoscience with spin-polarized electrons produced in high-order above-threshold ionization by a bicircular field.

Twisted particles as a new tool to study micro world phenomena and processes

Twisted or vortex particles are a new powerful tool to study atomic and molecular processes as well as phenomena that occur at the level of nano-objects. The main feature of such particles is that they carry a non-zero projection of orbital angular momentum along the beam propagation direction. The process of twisted electron scattering from diatomic molecule targets has been studied in this paper for the first time. The Yukawa potential is selected as a model potential. Numerical calculations are carried out for the case of scattering from a hydrogen molecule H2.