Filming movies of attosecond charge migration in single molecules with high harmonic spectroscopy

Abstract

Electron migration in molecules is the progenitor of chemical reactions and biological functions after light-matter interaction. Following this ultrafast dynamics, however, has been an enduring endeavor. Here we demonstrate that, by using machine learning algorithm to analyze high-order harmonics generated by two-color laser pulses, we are able to retrieve the complex amplitudes and phases of harmonics of single fixed-in-space molecules. These complex dipoles enable us to construct movies of laser-driven electron migration after tunnel ionization of N2 and CO2 molecules at time steps of 50 attoseconds. Moreover, the angular dependence of the migration dynamics is fully resolved. By examining the movies, we observe that electron holes do not just migrate along the laser polarization direction, but may swirl around the atom centers. Our result establishes a general scheme for studying ultrafast electron dynamics in molecules, paving a way for further advance in tracing and controlling photochemical reactions by femtosecond lasers.

Attochemistry Regulation of Charge Migration

“…In contrast, charge migration (CM) dynamics, as we define it here, is the specific case of a rapidly created localized electron density hole traveling across a molecule in a particle-like way. − CM dynamics are postulated to influence longer-time photochemical processes such as photosynthesis, photocatalysis, and light harvesting. ,, Furthermore, these dynamics are expected to modulate photochemical reactivity since the distribution of charge in a molecule influences nuclear motion . Since its discovery, there have been numerous theoretical studies of CM in small molecules using correlated methods ,,− and time-dependent density functional theory, ,,− along with some experimental studies using high harmonic generation and pump–probe ionization methods. ,, Organic aromatic molecules are especially promising since they support facile CM due to their conjugated π-electron system, ,,, within which the hole can be viewed as hopping between π-bonds. , Although holes may, during a CM event, move with a particular dominant frequency and thus superficially appear similar to a few-state beating, due to the complicated multistate nature of the initial state, CM is better described as a mode in the nonlinear dynamics sense of the word . As with all coherent dynamics, CM can persist, be enhanced, or be disrupted by nuclear motion, typically on the time scales of a few bond vibrations (sim 10 fs).…”

“… 1 , 13 , 24 Furthermore, these dynamics are expected to modulate photochemical reactivity since the distribution of charge in a molecule influences nuclear motion. 25 Since its discovery, there have been numerous theoretical studies of CM in small molecules using correlated methods 12 , 15 , 26 − 29 and time-dependent density functional theory, 8 , 11 , 30 − 34 along with some experimental studies using high harmonic generation and pump–probe ionization methods. 3 , 4 , 35 Organic aromatic molecules are especially promising since they support facile CM due to their conjugated π-electron system, 14 , 33 , 36 , 37 within which the hole can be viewed as hopping between π-bonds.…”

Attosecond probing and control of charge migration in carbon-chain molecule

“…At the time of recombination, the information of the electron wave packet of the cation is encoded in both the amplitude and phase of each generated harmonic. The intrinsic time-to-frequency mapping underlying HHG allows a temporal measurement of the electron dynamics with a resolution of tens to a hundred attoseconds [25][26][27][28][29][30]. Up to now, HHS has been well used for ultrafast molecular detection with both attosecond temporal [25][26][27][28][29][30] and ångström spatial resolutions [31][32][33].…”

Tailoring the waveshape of optical unipolar pulses in a multi-level resonant medium

“…Over recent years significant progress has been achieved in generation of ultra-short pulses up to fewcycle and subcycle duration in the optical range [1][2][3][4][5][6][7][8]. As the result, the possibilities to observe and even control different ultrafast processes in matter has become available [9][10][11][12][13].…”