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arXiv:0905.0786 · Journal of the Physical Society of Japan

Slow Molecules Produced by Photodissociation

Bum Suk Zhao, So Eun Shin, Sung Tai Park, Xingnan Sun, Doo Soo Chung

Published Sep 10, 2009• 7 citations
arXivHTMLPDF

Abstract

A simple method to control molecular translation with a chemical reaction is demonstrated. Slow NO molecules have been produced by partially canceling the molecular beam velocity of NO$_2$ with the recoil velocity of the NO photofragment. The NO$_2$ molecules were photodissociated using a UV laser pulse polarized parallel to the molecular beam. The spatial profiles of NO molecules showed two peaks corresponding to decelerated and accelerated molecules, in agreement with theoretical prediction. A significant portion of the decelerated NO molecules stayed around the initial dissociation positions even several hundred nanoseconds after their production.

DOI
10.1143/jpsj.78.094302

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A simple method to control molecular translation with a chemical reaction is demonstrated. Slow NO molecules have been produced by partially canceling the molecular beam velocity of NO2_22​ with the recoil velocity of the NO photofragment. The NO2_22​ molecules were photodissociated using a UV laser pulse polarized parallel to the molecular beam. The spatial profiles of NO molecules showed two peaks corresponding to decelerated and accelerated molecules, in agreement with theoretical prediction. A significant portion of the decelerated NO molecules stayed around the initial dissociation positions even several hundred nanoseconds after their production.
The use of photodissociation to produce cold, slow molecules from a molecular beam of cold, fast molecules is dubbed “photostop”. The essence of the scheme is thus: a pulsed molecular beam of a precursor molecule AB seeded in a noble carrier gas is crossed by a laser beam. The laser light dissociates AB, producing fragments A and B. These recoil from the dissociation site. The speed of the molecular beam and the wavelength of the laser beam are tuned so that the recoil velocity of those A molecules recoiling opposite to the molecular beam direction cancels out their initial velocity. This leaves a certain amount of A close to stationary in the laboratory frame. The photostop technique was first demonstrated with the production of cold, slow NO radicals by the photodissociation of nitrogen dioxide, both at Durham and in South Korea. The work described in this thesis began as a development of the NO photostop experiment, with an attempt made to magnetically trap NO. Latterly, the photostop project became part of the Millikelvin Molecules in a Quantum Array (MMQA) collaboration, the aim of which is the trapping of ten million cold, polar molecules. To this end, the photostop of SH radicals by the photodissociation of hydrogen disulphide was demonstrated.
Photostop is an accessible technique capable of producing atoms or molecules at a standstill in the laboratory frame. Starting with a NO2/Xe molecular beam with a mean velocity of 415 m s−1 and a longitudinal translational temperature of 6 K, NO2 molecules are photodissociated to yield NO(X ) fragments with a recoil speed equal to the molecular beam speed. The fraction of NO fragments that recoil opposite to the molecular beam are produced with a 6 K longitudinal velocity distribution centred at zero. The NO molecules are allowed to ‘evaporate’ from the probe volume by waiting for 10 µs and the molecules left behind are probed with a translational temperature of 1.6 K along the molecular beam axis and an estimated density of 107 cm−3 per quantum state. Through the choice of suitable precursors, the photostop technique has the potential to extend the list of atoms and molecules that can be slowed or trapped. It should be possible to accumulate density in a trap through consecutive loading of multiple pulses. †Current address: AECL Chalk River Laboratories, Deep River ON, K0J 1J0, Canada.
We demonstrate that chemical reactions in collisions of molecular beams can generally produce low-velocity molecules in the laboratory-fixed frame. Our analysis shows that collisions of beams may simultaneously yield slow reactant molecules and slow products. The reaction products are formed in selected rovibrational states and scattered in a specific direction, which can be controlled by tuning the kinetic energies of the incident beams and the angle between the beams. Our calculations indicate that chemical reactions of polar alkali-metal dimers are barrierless and we suggest that chemical reactions involving alkali-metal dimers may be particularly suitable for producing slow molecules in crossed beams.
Photodissociation of a molecule produces a spatial distribution of photofragments determined by the molecular structure and the characteristics of the dissociating light. Performing this basic reaction at ultracold temperatures allows its quantum mechanical features to dominate. In this regime, weak applied fields can be used to control the reaction. Here, we photodissociate ultracold diatomic strontium in magnetic fields below 10 G and observe striking changes in photofragment angular distributions. The observations are in excellent agreement with a multichannel quantum chemistry model that includes nonadiabatic effects and predicts strong mixing of partial waves in the photofragment energy continuum. The experiment is enabled by precise quantum-state control of the molecules.
The production of a translationally cold (T < 1 K) sample of bromine atoms with estimated densities of up to 10(8) cm(-3) using photodissociation is presented. A molecular beam of Br(2) seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms(-1), corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source.
The merging of molecular beam methods with those of accelerator physics has yielded new tools to manipulate the motion of molecules. Over the last few years, decelerators, lenses, bunchers, traps, and storage rings for neutral molecules have been demonstrated. Molecular beams with a tunable velocity and with a tunable width of the velocity distribution can now be produced, and are expected to become a valuable tool in a variety of physical chemistry and chemical physics experiments. Here we present a compact molecular beam machine, capable of producing 3D spatially focused packets of state-selected accelerated or decelerated molecules.