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arXiv:1002.3698 · Molecular Physics

Photostop: production of zero-velocity molecules by photodissociation in a molecular beam

Alexandre Trottier, David Carty, Eckart Wrede

Published Mar 5, 2011• 19 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1080/00268976.2010.550142

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This thesis is an expansion on previous work using the photostop technique for the production of near-zero velocity atoms and molecules. The goal is to produce stopped SH molecules and trap them in a permanent magnetic trap and the aim of this project was to construct a new experimental apparatus to accomplish this. During initial tests of the apparatus, the Rayleigh scattering cross-section of N2 was measured to provide a reference point for future experiments. The uncertainty and systematic errors in the measurements was such that denitive quantitative results of this were not be obtained at this stage. The emerging technique of cavity-enhanced laser-induced uorescence (CELIF) was used to perform absolute number density measurements of a molecular beam of SO2. CELIF was then applied to measuring the photostop of SD/SH. This showed that CELIF would not have the required sensitivity to measure the trapped SD/SH molecules due to issues of stray light from the lasers. As a result of this we elected to use resonance-enhanced multi-photon ionisation (REMPI) as an alternative. We devised and constructed a novel ion extraction system for use in performing REMPI, which was based on a time-of- ight mass spectroscopy system, but utilising the magnets themselves as electrodes, as well as some ion lensing components. This was initially tested using Xe, showing a strong signal and good mass resolution. Using this, the photostop of SH and S was measured showing that the detection apparatus is able to distinguish signal over a range of 9 orders of magnitude. However, despite this sensitivity, the trapping of these stopped molecules could not initially be demonstrated as the signal from these stopped molecules was obscured by signal from the inadvertent dissociation of the background parent molecules by the probe laser. More recent measurements in the group have directly addressed this issue with background subtraction and the results have now demonstrated the trapping of SH. Signicant headway has been made in the demonstration of the trapping of SH produced by photostop. From the results produced using REMPI the detection limit has improved signicantly over the prior experiments and very recent measurements have successfully demonstrated the trapping of SH.

•Jan 1, 2016•0 citations
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.
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.
This thesis describes the development of a relatively novel technique for the gen- eration and subsequent trapping of cold species. Molecules in a pulsed supersonic expansion are photolysed, such that the centre-of-mass velocity vector of one of the fragments is equal in magnitude but opposed in orientation to the lab-frame velocity of the precursor molecule. This technique, known as ‘Photostop’, leaves a fraction of the fragments with a narrow velocity distribution, centered around zero velocity in the lab-frame. They can be shown to have zero velocity by changing the time between photodissociation and ionisation; fragments with a high kinetic energy will leave the ionisation volume prior to interrogation. The underlying velocity distribu- tion is uncovered by using the velocity-map imaging technique, and the temperature of the fragments can be determined.The method was originally optimised for the molecular case. Cold NO has been produced from the dissociation of NO₂ molecules, and a single rotational state has been shown to remain in the ionisation volume 10 μs after dissociation, implying a sample temperature of 1.17 K. Using the optimised experimental conditions de- rived from the velocity cancellation of NO, the atomic case is demonstrated for the dissociation of Br₂ to give zero-velocity Br fragments. The Br atoms are seen for delay times in excess of 100 μs, showing the greater applicability of the method to the atomic case. The temperature of the residual atoms is shown to be in the milliKelvin regime, as determined through detailed Monte Carlo simulation of the motion of the stopped atoms. The possibility of trapping the ultracold Br atoms in a magnetic field is explored, and a quadrupolar trap created between two per- manent bar magnets is demonstrated to confine the atoms spatially, within the ion extraction optics, for delays in excess of 1 ms.The Photostop technique is intended to be a stepping stone on the way to widening the number of chemical species available for study in the ultracold regime. The possibility of improvements to the experiment is considered, in order to increase the efficiency of the experiment such that the number density becomes high enough to be viable as a source of atoms for use in cold chemical reactive studies. The possibility of extending the method so as to be used as a tunable velocity source of atoms is also discussed.
Successful magnetic trapping of both SH at a density of 2.4 x 10^5 cm^-3 and atomic oxygen at a density of 3000 +/- 900 cm^-3 has been demonstrated with respective trapping lifetimes of 40 +/- 10 ms and 82 +/- 3 ms inside a 0.41 T deep, anti-Helmholtz configuration magnetic trap. To achieve trapping of these species, a stable precursor was cooled by supersonic expansion in a molecular beam and photodissociated in such a way as to match and oppose a photofragments recoil speed to the precursors molecular beam speed. This process is called photostop. Velocity-map images have been taken of photostop of atomic oxygen from photodissociation of NO2 in a molecular beam. They show that atomic oxygen can be photostopped from a variety of NO photodissociation co-fragments with a suitably broad molecular-beam velocity-distribution. Although photostop is simple and cheap to use, the trapping lifetimes and densities that have been achieved in this thesis are not competitive compared to other methods of trapping similar molecular species. The main problem that presents universally in photostop experiments is due to the large number of fast molecules from the supersonic expansion that pass through the trap after the photostop event. Only a small fraction of the molecules in the molecular beam are stopped and the remaining ones cause significant trap loss through high energy elastic collisions with trapped species. Work has been carried out to develop the cavity-enhanced laser-induced-fluorescence (CELIF) technique to be used more widely in the cold molecules field. The technique was employed unsuccessfully to measure photostopped SH radicals but was used elsewhere to measure cold SD radicals after Stark deceleration. In addition, the technique has been initially developed for use inside magnetic traps. Clusters of NO.Ar(n) have been studied using [1+1'] REMPI to assess the viability of a new soft-ionisation technique for molecular clusters of atmospheric interest. This work did not show conclusive evidence that [1+1'] REMPI would reduce cluster fragmentation and displayed some interesting but poorly understood data.
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.
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.
This thesis is an expansion on previous work using the photostop technique for the production of near-zero velocity atoms and molecules. The goal is to produce stopped SH molecules and trap them in a permanent magnetic trap and the aim of this project was to construct a new experimental apparatus to accomplish this. During initial tests of the apparatus, the Rayleigh scattering cross-section of N2 was measured to provide a reference point for future experiments. The uncertainty and systematic errors in the measurements was such that denitive quantitative results of this were not be obtained at this stage. The emerging technique of cavity-enhanced laser-induced uorescence (CELIF) was used to perform absolute number density measurements of a molecular beam of SO2. CELIF was then applied to measuring the photostop of SD/SH. This showed that CELIF would not have the required sensitivity to measure the trapped SD/SH molecules due to issues of stray light from the lasers. As a result of this we elected to use resonance-enhanced multi-photon ionisation (REMPI) as an alternative. We devised and constructed a novel ion extraction system for use in performing REMPI, which was based on a time-of- ight mass spectroscopy system, but utilising the magnets themselves as electrodes, as well as some ion lensing components. This was initially tested using Xe, showing a strong signal and good mass resolution. Using this, the photostop of SH and S was measured showing that the detection apparatus is able to distinguish signal over a range of 9 orders of magnitude. However, despite this sensitivity, the trapping of these stopped molecules could not initially be demonstrated as the signal from these stopped molecules was obscured by signal from the inadvertent dissociation of the background parent molecules by the probe laser. More recent measurements in the group have directly addressed this issue with background subtraction and the results have now demonstrated the trapping of SH. Signicant headway has been made in the demonstration of the trapping of SH produced by photostop. From the results produced using REMPI the detection limit has improved signicantly over the prior experiments and very recent measurements have successfully demonstrated the trapping of SH.