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# Rotationally cold ( > 99% J = 0) OH − molecular ions in a cryogenic storage ring

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We store a 10 keV OH⁻ ion-beam at 13.5 ± 0.5 K in one of the DESIREE storage rings. Using photodetachment thermometry we measure the effective relative photodetachment cross section at different storage times and determine the rotational temperature of the ions to be 13.4 ± 0.2 K in agreement with the macroscopic temperature. A model cross section in the threshold range taking into account the formation of excited neutral OH molecules is calculated as a function of rotational temperature in order to justify the use of the rotational thermometry method developed earlier by the group of Roland Wester at Innsbruck University in the present case. In addition, we apply a selective photodetachment technique to produce an ion beam with more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J = 1 rotational level is measured to be 145 ± 28 s.
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Journal of Physics: Conference Series
PAPER • OPEN ACCESS
Rotationally cold (
>
99%
J
= 0) OH
molecular ions in a cryogenic
storage ring
To cite this article: Gustav Eklund et al 2017 J. Phys.: Conf. Ser. 875 012016
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ICPEAC2017 IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
Rotationally cold (>99% J= 0) OHmolecular ions
in a cryogenic storage ring
Gustav Eklund1, Kiattichart Chartkunchand1, Emma K Anderson1,
Magdalena Kami´nska1,2, Nathalie de Ruette1, Richard D Thomas1,
Moa K Kristiansson1, Michael Gatchell1, Peter Reinhed1, Stefan
Ros´en1, Ansgar Simonsson1, Anders K¨allberg1, Patrik L¨ofgren1, Sven
Mannervik1, Henning Zettergren1, Henrik Cederquist1and Henning
T Schmidt1
1Stockholm University, Department of Physics, Stockholm 10691, Sweden
2Institute of Physics, Jan Kochanowski University, 25-369 Kielce, Poland
E-mail: gustav.eklund@fysik.su.se
Abstract. We store a 10 keV OHion-beam at 13.5±0.5 K in one of the DESIREE storage
rings. Using photodetachment thermometry we measure the eﬀective relative photodetachment
cross section at diﬀerent storage times and determine the rotational temperature of the ions to
be 13.4±0.2 K in agreement with the macroscopic temperature. A model cross section in the
threshold range taking into account the formation of excited neutral OH molecules is calculated
as a function of rotational temperature in order to justify the use of the rotational thermometry
method developed earlier by the group of Roland Wester at Innsbruck University in the present
case. In addition, we apply a selective photodetachment technique to produce an ion beam with
more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J= 1
rotational level is measured to be 145 ±28 s.
1. Introduction
In experiments on molecular ions it would be of great advantage if all ions are populated in the
same quantum state. However, most available ion sources usually produce hot ions occupying
a large number of vibrational and rotational states. To relax these degrees of freedom many
experiments have been performed where ions are stored in traps and storage rings. Relaxation of
the vibrational degrees of freedom has been studied extensively in room temperature experiments
with storage times on the order of tens of seconds. For rotational relaxation the time scales are
much longer [1] and at room temperature, a large number of rotational states are occupied.
Recent development in cryogenic storage techniques [2–5] allows for ion-beam storage times of
hours [6] and has opened up new possibilities to study rotationally cold molecular ions.
In this work we present a study of rotational relaxation of the OHmolecular ion using the
photodetachment thermometry technique developed by Otto et al. [7] in one of the DESIREE
storage rings. The hydroxyl anion, OH, is a suitable molecular ion for photodetachment
studies with its well separated rotational levels and large rotational constant B= 18.7354(16)
cm1[8]. Several experiments set out to study cold OHhas been performed recently. Otto et
al. [7] performed photodetachment thermometry on OHstored in a 22-pole radiofrequency trap
using buﬀer gas cooling with He. They found that the rotational temperature agreed well with
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IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
the macroscopic temperature of their environment except at the lowest temperatures where the
rotational temperature always remained higher for reasons that were not clear at the time [7].
Possible explanations for the lack of cooling to below about 20 K in the buﬀer gas trap was
further investigated by Endres et al. [9] but again no ﬁrm conclusion was reached. Recently,
Meyer et al. [10] studied the rotational relaxation of OHin the Cryogenic Storage Ring (CSR)
and found the eﬀective radiative temperature of the blackbody ﬁeld in the ring to be 15.1±0.1
K although the main part of their ring was substantially colder than this. They also measured
the intrinsic lifetime of the ﬁrst three excited rotational levels of OH[10].
2. Experiment
Here, we use one of the DESIREE storage rings, shown schematically in Figure 1, to study the
cooling of OHions in a 10 keV beam. The storage rings are contained in a single cryogenically
cooled chamber at 13.5±0.5 K with a residual gas density of 104H2per cm3. The OHions
are produced in a Cesium sputter ion source, accelerated to 10 keV and mass selected by a
bending magnet. A bunch of 1-10 million ions is injected into the ring. During the present
experiment, a 1/e storage lifetime of approximately 10 minutes is achieved. Two laser and
two detection systems are used. The ﬁrst is a tunable cw Ti:Sapphire laser overlapping the
ion beam collinearily in the straight section on the injection side of the ring. The neutral
OH produced from photodetachment hit a glass plate emitting secondary electrons which are
accelerated and detected by an MCP. The second system consists of a tunable pulsed OPO
interacting perpendicularly with the ion beam on the opposite straight section. The neutral
particles are detected by an Imaging Detector consisting of a triple stack MCP and phosphor
screen viewed by a CMOS camera and a photomultiplier tube.
Imaging
Detector
MCP Collinear
cw Laser Beam
Perpendicular Pulsed
OPO Beam
Glass Plate
OH
OH Injection
OH
e
Figure 1. Schematic of one of
the DESIREE storage rings. Lasers
and detectors for rotational ther-
mometry and manipulation studies
are indicated.
3. Photodetachment cross section
The eﬀective relative photodetachment cross section is modeled by [7]
σpd
eﬀ (E, T )X
J00
X
J0
I(J00, J 0)P(J00, T )(EJ00 ,J 0)p(1)
where Eis the energy of the incoming photon, J00 is the initial rotational state of the ion, and J0is
the ﬁnal rotational state in the neutral molecule. I(J00 , J 0) is an intensity factor for the transition
J00 J0[8, 11], P(J00 , T ) is the population in level J00,J00,J 0is the threshold energy for the
photodetachment transition, and p= 0.28 is an exponent for the cross section near threshold [12].
The allowed transitions are given by electric dipole selection rules ∆J=3/2,1/2,+1/2,+3/2
for the case of OH[8, 11]. The eﬀective relative photodetachment cross section is dependent
on the populations P(J00 , T ) and hence the temperature Tof the ions. Assuming thermal
equilibrium the populations P(J00 , T ) are given by P(J, T )(2J+ 1) exp [J(J+ 1)B/kBT]
where kBis the Boltzmann constant. Using the experimental thresholds J00 ,J 0[13], σpd
eﬀ (E, T )
can be modeled using Equation 1. Figure 2 shows calculated σpd
eﬀ (E, T ) curves for photon
energies, E, near the electron aﬃnity of OH for diﬀerent rotational temperatures, T. At high T
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IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
a large number of transitions to excited states in the neutral OH are contributing to σpd
eﬀ (E, T ).
As can been seen in Figure 2, σpd
eﬀ (E, T ) is completely dominated by three transitions (from
J00 = 0,1,2 to the ground state J0= 3/2 of OH) at T= 15 K.
To determine the rotational temperature of the ions we measure σpd
eﬀ (E) as a function of
photon energy using the cw laser, correcting for the Doppler shift resulting from the collinear
geometry. The experiment is repeated for diﬀerent storage times as shown in Figure 3. At photon
energies below 1.8137 eV the measured σpd
eﬀ (E) is constant which is attributed to contamination
from 17Oin the ion beam. The diﬀerences in levels between the measurements in this energy
range are due to variations in ion source conditions. Three thresholds are clearly distinguishable
in the data which is consistent with the model in Figure 1. To extract the populations P(J00)
Equation 1 is ﬁtted to the data using the three dominant transitions and a constant to account
for the 17Ocontamination. After 10 minutes of storage P(0) = 93.9±0.2 % and under the
assumption of thermal equilibrium this corresponds to a temperature of 14.1±0.2 K.
Figure 2. Modeled σpd
eﬀ (E, T ) for diﬀerent
rotational temperatures. As the temperature
decreases transitions from J00 = 0,1,2 to the
ground state J0= 3/2 dominates.
Figure 3. Measured σpd
eﬀ (E) for 50, 300
and 600 s of storage. The thresholds for the
dominating transitions are marked by dashed
lines. The curves are ﬁts to Equation 1.
4. Probing as function of time and selective photodetachment
To determine if thermal equilibrium is reached the populations need to be measured for longer
times. From the measurements described above it is clear that P(2) becomes negligible at
long storage times, thus for this measurement it is suﬃcient to consider J= 0 and J= 1. Using
the pulsed OPO two measurements are made, at λ= 677 nm (E= 1.8314 eV) and λ= 679 nm
(E= 1.8260 eV) for photodetachment of J0 and J1 respectively. Since the form of σpd
eﬀ (E)
is known from Equation 1 the populations P(0) and P(1) can be extracted. The triangles in
Figure 4a show P(1) as a function of storage time.
In addition to probing the rotational relaxation of the ions we also apply the cw laser at
λ= 679 nm for selective photodetachment of ions with J1 (diamonds in Figure 4a). The
result is a faster decay and a lower equilibrium population than without the depletion laser.
The circles in Figure 4a show a measurement where the cw laser is turned on initially but
switched oﬀ after some time causing repopulation of the J= 1 level due to interaction with the
surrounding blackbody radiation. Figure 4b shows a similar measurement where the overlap
between the laser and ion beams is better optimized resulting in a more eﬃcient depletion.
The lowest P(1) in this case is 0.9±0.1% and >99% of the ions are in the rotational ground
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state. There is a diﬀerence in the asymptotic value of P(1) between the spontaneous decay
and after repopulation. This is because the cw laser induce photodetachment of 17Oand
thus removes this beam contamination permanently. The repopulation measurement provides a
better estimate of the asymptotic P(1) and the ﬁnal result is given by the weighted average of
the two repopulation measurements of 5.1±0.3%, which yields a temperature of 13.4±0.2 K of
the stored ensemble of OHions. Furthermore by considering the repopulation measurements in
a 13.4±0.2 K blackbody radiation ﬁeld, we extract the intrinsic lifetime of the J= 1 rotational
level A1
10 = 145 ±28 s. [14]
Figure 4. Fractional population
in J= 1 as a function of storage
time. (a) Triangles: spontaneous
relaxation. Diamonds: cw laser
depleting J1. Circles: cw
laser depleting J1 until 440
(380) s, J= 1 repopulates due to
interaction with the blackbody
ﬁeld. (b) A measurement with
improved overlap between the cw
laser and ion beam.
5. Conclusions
Using photodetachment thermometry we have measured the asymptotic rotational temperature
of the OHion beam to be 13.4±0.2 K, which demonstrates thermal equilibrium with the
13.5±0.5 K storage ring. Using selective photodetachment we have produced an ion beam with
>99% of the ions in the rotational ground state. This technique will be beneﬁcial for studies of
ion interactions with photons, electrons, neutrals and other ions, as well as action spectroscopy
of ions at interstellar temperatures. We measure the intrinsic lifetime of the J= 1 rotational
level to be 145 ±28 s, which is about two standard deviations shorter than the corresponding
result of Meyer et al. [10].
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... The s-wave threshold detachment of OH − has been extensively studied through photodetachment [27,28] and deviations from the Wigner threshold law have been observed due to the electron-dipole interaction [29,30]. With its large rotational constant, OH − has also been used to probe the rotational thermalization by buffer gas cooling [26,31] and in cryogenic storage rings [32,33]. In buffer gas at low temperatures, significant deviations of the rotational temperature from the thermal bath have been seen [34]. ...
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