Direct IR excitation in a fast ion beam: application to NO- photodetachment cross sections
© Otto et al.; licensee Springer on behalf of EPJ. 2014
Received: 5 December 2013
Accepted: 5 March 2014
Published: 29 April 2014
Optical access to a travelling ion packet is required in many ion beam experiments that study ion-photon interactions.
An approach is described for carrying out direct infrared excitation of a fast ion beam that uses an optical-quality reflective beam blocker to illuminate a counter propagating pulsed ion beam in a collinear configuration. This arrangement provides optical access along the axis of ion beam propagation by placing a mirror in the beam path at a 25 degree angle. The ion packet is bumped over the mirror, which is also used to block fast neutral particles produced during ion beam acceleration that also propagate along the beam path.
The efficiency of this setup is demonstrated in a photodetachment experiment on NO- anions, where a photoinduced depletion of up to 90% of the beam is achieved in a single laser shot. To demonstrate the application of this configuration, the relative photodetachment cross section for NO- has been measured in the range of 2800 – 7200 cm-1. The measured relative cross section shows a set of sharp peaks that are identified as vibrational autodetachment resonances.
The new setup paves the way for future experiments where parent anionic species are vibrationally excited via direct infrared excitation first and undergo photodetachment/photodissociation in a subsequent step.
33.15.Ry; 33.80.Eh; 33.80.-b
KeywordsInfrared excitation Fast ion beam Nitric oxide anion photodetachment
Optical access to a travelling ion packet is required in many ion beam experiments that study ion-photon interactions. Ion storage rings can provide optical access tangentially to the beam path, thereby enabling lifetime measurements of metastable ions . Multipass cell [2, 3] or resonator  arrangements are used to increase the overlap between a short laser pulse and a molecular beam. Developments in manufacturing microchannel plate (MCP) ion detectors have enabled photofragmentation experiments where a detector featuring a center hole allows laser access to a fast moving ion packet . Cluster predissociation studies often make use of a spatial and temporal focus of the ion bunch to maximize the overlap with a laser that crosses the beam from the side . However, due to conservation of phase space, such a focus will always create an axial energy spread of the beam that might be undesirable if the kinetic energy release of the fragments is of interest. The work presented here was inspired by the need to illuminate an outstretched ion packet (~30 cm) in a completely collinear ion beam experiment, in which photoelectrons and photofragments created from a fast moving ion beam are collected in coincidence (PPC) .
The fast-beam apparatus used in these studies features a plasma discharge ion source where anions are created in a supersonic expansion at a 10 Hz duty cycle. The ions pass through a skimmer together with the gas jet and can be accelerated up to 10 keV, with an energy spread of less than 0.1 eV, before being re-referenced to ground while traveling through a cylindrical electrode. The length of this cylinder (30 cm) determines the spatial extension of the ion packet. After a flight distance of ~1 m the ion packet is injected into an electrostatic ion trap (EIBT), where it is repetitively probed perpendicularly with the pulsed output from a 1 kHz Ti:Sapphire laser system for studies of photodetachment and dissociative photodetachment processes. As part of an effort to expand the present set of experiments towards direct infrared excitation of the parent anionic species prior to the photodetachment step, it has become essential to irradiate the entire outstretched ion packet with the output of a 10 Hz infrared (IR) optical parametric oscillator/optical parametric amplifier (OPO/OPA) laser system before it enters the EIBT. In addition, such an arrangement opens up the possibility to study photodetachment processes in weakly bound anionic species.
Here we demonstrate a simple configuration where a single gold mirror is placed in the ion beam, with the surface normal of the mirror at an angle of 25 degrees relative to the ion beam propagation direction. The pulsed IR laser light enters the vacuum through a viewport located on the side of the vacuum chamber such that it forms an angle of 50 degrees with the beam axis. The laser pulse reflects off the gold mirror and illuminates the entire ion bunch in a single shot. The ions are then electrostatically bumped over the mirror and corrected to the ion beam axis to continue travelling towards the EIBT. As a secondary effect, the gold mirror acts as a beam blocker for fast neutral particles in the beam, preventing this source of background from striking the neutral particle detector used in the PPC experiments. This also ensures ultra-high vacuum conditions in the EIBT and detection regions. The performance of the new setup was demonstrated in a photodetachment experiment, making use of the small electron affinity (EA = 26 ± 5 meV)  of the NO molecule to directly deplete ions from a fast ion beam. A measurement of the wavelength dependence of the depletion, which is proportional to the photodetachment cross section, reveals sharp resonance features that are associated with vibrational transitions in the NO- anion, followed by vibrational autodetachment.
Results and discussion
Design and performance of the reflective beam blocker
The shape of the cross section makes it obvious that two different processes are observed in the experiment. The first process is a bound-free transition between the molecular anion and a neutral NO molecule plus an electron in the continuum. This process is responsible for the continuous part of the spectrum that slowly decreases with increasing photon energy. On top of that resonant peaks are observed that are identified as vibrational transitions induced in the NO- molecule by IR absorption, followed by autodetachment.
Direct photodetachment of NO-
with the threshold energy E0 and the angular momentum l of the outgoing electron. The more complex zero core contribution (ZCC) model  has been used to describe the shape of the cross section above threshold for atomic systems. While the Wigner law is well suited to describe the rising cross section behavior close to the threshold, the ZCC model also reproduces a decaying cross section at higher energies. Al-Za’al et al. applied the model to NO- photodetachment and predicted a sharp rise in the cross section at 0.507 eV (4090 cm-1), which is associated with the channel to produce NO (v = 2) opening up . Surprisingly they could not verify this threshold experimentally. Instead a continuous spectrum without sharp increases was observed that slowly decreased over the experimental range. The authors based their analysis on the electron affinity for the NO- (v = 0) → NO (v = 2) transition measured by Siegel et al., and added a rotational correction of 12.5 meV. Based on those values another rise in the cross section is expected at 0.732 eV (5903 cm-1) where the NO (v = 3) state becomes accessible. More recent photoelectron spectroscopy experiments suggest values of 0.488 eV and 0.714 eV for the NO (v = 2) and NO (v = 3) electron affinity relative to NO- (v = 0) .
In the results reported here a slowly decaying cross section attributed to direct photodetachment that is accompanied by several sharp resonance features is observed. The continuous part of the cross section decays by a factor of 10 over the measured range. Upon closer inspection, two regions in the spectrum at 4100 cm-1 and 5900 cm-1 can be identified where a sudden increase in the cross section is observed. Both of these features are assigned to the opening of new product channels, leading to NO (v = 2) and NO (v = 3) products respectively, as indicated in the figure. The cross section at the (v = 2) threshold rises by almost a factor of two, in accordance with the predictions from the ZCC model . It is interesting to note that above this threshold the cross section reaches a maximum after only 100 cm-1 before starting to decrease again. The (v = 3) threshold is not analyzed here, since it is located close to one of the resonance features that will be discussed in the next section.
Photoinduced Vibrational Autodetachment of NO-
Peak positions observed in the photodetachment cross section of NO-
Energy (cm -1 )
Width (cm -1 )
1284 ± 10
95 ± 15
3Σ¯ (v = 1)
3687 ± 2
28 ± 3
3Σ¯ (v = 3)
5290 ± 3
41 ± 3
3Σ¯ (v = 4)
5976 ± 13
108 ± 24
1Δ (v = 0)
6355 ± 10
115 ± 16
3Σ¯ (v = 5)
The observed intensity of the resonance peaks is largest for the (v = 3) resonance and decreases for higher vibrational excitation, which can be understood in terms of a decreasing Franck-Condon overlap . The fact that the NO- (v = 3) level is nearly degenerate with NO (v = 2) also contributes to the high intensity of the (v = 3) resonance. Finally, it is interesting to note that the spacing found between the (v = 3) and (v = 4) resonances is significantly larger (ΔE = 1600 cm-1) than that between the (v = 4) and (v = 5) feature (ΔE = 1100 cm-1). The resonance features detected in electron scattering show a consistent spacing of about 0.16 eV (1300 cm-1) at least up to (v = 9), pointing to only weak anharmonicity in the NO- potential. However, it has to be pointed out that the underlying mechanism in those experiments is different in nature than the direct bound-bound transition probed in vibrational autodetachment. Also, it cannot be ruled out that rotational effects might play an important role in the autodetachment process . An analysis of such effect is beyond the scope of this work.
A new experimental setup has been presented here that allows direct IR excitation in a completely collinear pulsed ion beam experiment that does not feature any other optical access along the ion beam path. This has been achieved by placing an optical-quality mirror in the beam path at an angle that allows coupling in an IR laser from the side of the setup. Photodetachment-induced depletion of an NO- beam was used to demonstrate the overlap that can be achieved between the laser pulses and a fast moving ion packet. Furthermore, measurements of the relative photodetachment cross section of the NO- molecule in the range of 2800 – 7200 cm-1 were made. It was found that the cross section in this range is a combination of direct photodetachment and vibrational autodetachment. The observed autodetachment resonances are within the range of previous experimental results from electron scattering experiments.
This new setup paves the way for future PPC studies with vibrationally excited molecules. In these experiments molecular anions will be prepared in specific vibrational states before entering the ion beam trap, and the fragmentation dynamics in dissociative photodetachment processes will be studied. The systems amenable to study by this approach will in general be strongly bound anions where the lifetimes of the excited vibrational modes are long enough to allow for transferring the ions over the mirror and carrying out PPC experiments on a millisecond timescale.
To demonstrate the capabilities of the reflective beam blocker design for coupling a light source with a travelling ion packet, a photodetachment experiment using a beam of NO- molecules was carried out. These experiments exploit the low electron affinity of NO- that allows for efficient photodetachment at wavelengths between 2 – 5 μm. The NO- anions were generated from a 10 Hz pulsed discharge (20% N2O seeded in a 1:2 He/Ne mixture, 20 psi stagnation pressure). Typical rotational temperatures for this ion source have been measured to be 50 – 100 K using near threshold photodetachment of OH- in a different set of experiments. The ions were accelerated to 7 keV before approaching the reflective beam blocker. The ion signal was monitored 2.5 m behind the beam blocker using an off-axis MCP ion detector. Tunable IR laser pulses from a 10 Hz Nd:YAG (Surelite III EX) pumped OPO/OPA system (LaserVision, 5 ns FWHM, 3 cm-1 bandwidth) were coupled into the approaching ion packet at a time delay synchronized with the pulsed ion source. A typical output power between 100 and 300 mW can be achieved over the wavelength range covered in this work. The depletion of the beam caused by photodetachment of the NO- was derived from consecutively measuring the IR on and IR off ion signals (denoted as NIR and N0 respectively) at a given wavelength. To acquire each signal an average over 32 source cycles was recorded before the status of the IR laser was switched (IR on/off).
Photoelectron photofragment coincidence
Electrostatic ion beam trap
Zero core contribution.
This work was supported by the U.S. Department of Energy under grant number DE-FG03-98ER14879. RO thanks the German Academic Exchange Service (DAAD) for a postdoctoral research fellowship. AWR and JSD acknowledge support from GAANN grant P200A120223 from the U.S. Dept. of Education.
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