Abstract
Echo in mountains is a well-known phenomenon, where an acoustic pulse is mirrored by the rocks, often with reverberating recurrences. For spin echoes in magnetic resonance and photon echoes in atomic and molecular systems, the role of the mirror is played by a second, time-delayed pulse that is able to reverse the flow of time and recreate the original impulsive event. Recently, alignment and orientation echoes were discussed in terms of rotational-phase-space filamentation, and they were optically observed in laser-excited molecular gases. Here, we observe hitherto unreported fractional echoes of high order, spatially rotated echoes, and the counterintuitive imaginary echoes at negative times. Coincidence Coulomb explosion imaging is used for a direct spatiotemporal analysis of various molecular alignment echoes, and the implications to echo phenomena in other fields of physics are discussed.
3 More- Received 31 July 2016
DOI:https://doi.org/10.1103/PhysRevX.6.041056
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Molecules such as nitrogen or oxygen in the atmosphere are freely rotating and randomly oriented in space. However, if these molecules are excited by laser pulses that are short compared to their rate of rotation, they can be aligned in space: For a very short instant, the molecular axes point in a specific direction defined by the laser, and then the molecular orientation is randomized again. Our team has previously shown that if, after some delay, the molecules are exposed to another pulse, the second pulse serves as a sounding board, causing a series of periodic alignment echoes to reverberate for quite some time, just like an echo in the mountains. Here, we extend this finding and report a gamut of new echo phenomena, including fractional echoes that repeat themselves at fractions of the usual echo period, rotated echoes that shoot in a direction controlled by polarization of the laser pulses, and counterintuitive imaginary echoes that appear at negative times.
Using coincidence Coulomb explosion imaging of and molecules in a vacuum chamber, we investigated how very short laser pulses could modify molecular angular distribution by applying a torque to them. The gases were excited by two short laser pulses (sometimes with different polarization angles), and a third strong laser pulse was used to explode the excited molecules and to image the angular distribution of the molecular fragments.
We expect that a generic mechanism behind the alignment echoes may have implications in many fields of physics, leading to a number of useful applications.