Use of Oxygen Gas in the Low-TemperatureTime-Resolved EPR Experiments

Vinayak Rane • Krishnendu Kundu • Ranjan Das

Received: 8 February 2013 / Revised: 2 April 2013 / Published online: 10 May 2013

� Springer-Verlag Wien 2013

Abstract A large transient microwave signal seen in low-temperature time-

resolved electron paramagnetic resonance (TREPR) experiments is attributed to the

presence of nitrogen as a flushing gas, when pulses of a 266- or 248-nm laser light is

used for photolysis. We report here that, using oxygen as the flushing gas, this

transient can be largely removed. Based on the studies using 355 nm laser light and

also nitrous oxide as the flushing gas, photoelectron emission from the inner walls of

the microwave cavity is proposed to be the origin of this transient, and the electron

attachment to oxygen gas is the mechanism of its removal. Using oxygen as the

flushing gas, recording of TREPR spectra at low temperatures as well as very close

to the laser pulse of 266 or 248 nm is possible.

1 Introduction

Time-resolved electron paramagnetic resonance (TREPR) experiments showing

electron spin polarization are known to give a detailed insight into photophysical

and photochemical pathways induced by a light pulse. Through such experiments,

one can observe important spin-dependent interactions that are usually not

observable in optical studies. The temperature of the reactants affects most of the

kinetic parameters involved in the dynamics of the system. Thus, a temperature-

dependent study often leads to a much better insight into the spin-dependent

processes initiated by the light pulse.

Low-temperature steady-state EPR experiments, above liquid nitrogen temper-

ature, are usually carried out by passing cold nitrogen gas around a static sample

placed inside a variable temperature dewar insert (such as model WG-821-Q of

Wilmad Glass, NJ, USA), kept in the microwave cavity, to cool the sample to a

V. Rane � K. Kundu � R. Das (&)

Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India

e-mail: ranjan@tifr.res.in

123

Appl Magn Reson (2013) 44:1007–1014

DOI 10.1007/s00723-013-0455-9

Applied

Magnetic Resonance

desired temperature. In contrast to such static samples, time-resolved EPR

experiments of liquid samples often involve passing the liquid through a sample

cell (such as a flat cell model WG-812-Q of Wilmad Glass, NJ, USA) placed inside

the microwave cavity for cooling the sample to the desired temperature. Flowing the

sample is necessary to minimize the depletion of the reactants and accumulation of

products due to photolysis, which may interfere with the chemistry of the reactants

of interest. Different laboratories have adopted different ways of cooling the sample

while it flows through the microwave cavity. One method involves passing the

solution through a heat exchanger dipped in a low-temperature bath that is

maintained at a constant temperature, thus allowing the sample to reach the desired

temperature inside the cavity. In the second method, a special variable temperature

dewar insert is used (such as model WG-865-Q of Wilmad Glass, NJ, USA) in

which the solution can flow at a slow rate. Cold nitrogen gas flowing through the

dewar insert cools the sample to the desired temperature, in the same way as the

conventional variable temperature continuous-wave EPR experiments are done. In

both cases, the microwave cavity is flushed with dry nitrogen gas to avoid

condensation of moisture inside the cavity.

When TREPR experiments are performed at the ambient temperature, a sharp

spike of microwave transient signal lasting for a few hundred nanoseconds,

followed by a slow decay, is seen after the incident laser pulse. The slowly decaying

signal is attributed to the thermal disturbance in the microwave cavity due to the

absorption of laser energy by the sample. We have observed that this effect becomes

less pronounced with the lowering of temperature of the sample. Using a boxcar

averager while recording the TREPR spectra as a function of magnetic field, the

sharp spike is usually ignored. The gate of the boxcar is generally opened at delay

time of not less than about 200–300 ns after the laser pulse, when the sharp transient

has largely decayed.

During the course of our work with various laser lights during the past two

decades, we have seen that the presence of nitrogen flushing gas in the cavity

produces a strong transient microwave signal that lasts for several microseconds.

This effect could be seen when the wavelength of the laser light was less than

308 nm, for example, 266 nm (fourth harmonic of Nd:YAG laser) and 248 nm (KrF

excimer laser) [1–3]. The intensity of this transient completely masks any residual

transient EPR signal, so that no TREPR spectrum can be recorded in its presence.

Similar transient signals, but of smaller intensity and shorter duration, were also seen

when test experiments were carried out with a 308-nm laser (XeCl excimer laser).1

In this article we describe our finding that using oxygen gas to flush the cavity

largely removes this transient. In the published scientific literature, we could not

find any mention of the use of oxygen gas for low-temperature TREPR

experiments.2 By examining the dependence of the transient signal on the laser

1 Test experiments were carried out in the laboratory of Prof. G. Grampp, Graz University of

Technology, Graz, Austria.2 During a meeting (January 2012), Prof. G. Grampp, Graz University of Technology, Graz, Austria, told

us that he had read the Ph. D. thesis of Luis Miguel Francisco Jent, University of Zurich (1990), in which

the author mentioned observing strong transients in the presence of nitrogen, but not in the presence of

oxygen. However, no detailed investigation was carried out there.

1008 V. Rane et al.

123

wavelength and fluence, we showed that ionization from the surface of the

microwave cavity and quenching of the ejected electrons by oxygen gas could

explain all the observation. Because of its ease and simplicity, we expect this

method to be useful to the scientific community at large for recording TREPR

spectra at low temperatures, and at a time close to the laser pulse, when lasers of 266

or 248 nm is used.

2 Results and Discussions

The X-band TREPR spectrometer, used in this study, was constructed in our

laboratory. All the details of its initial construction have been described in Ref. [4],

and its subsequent modifications have been given in Ref. [1]. In the present version,

we have used a transient recorder model Compuscope 12100 (Gage-Applied,

Canada) with a sampling rate of 10 ns per sample. The light source was either a KrF

excimer laser (Coherent model Compex Pro 110F) giving 248 nm pulse or an

Nd:YAG laser (Quantel model YG981C) giving 355 and 266 nm pulses. The laser

light was not focussed. The laser light entered the microwave cavity through a small

hole of 5 mm diameter. The two types of arrangements for cooling the samples to

low temperatures are described below.

2.1 Cooling the Sample Outside the Cavity

In this technique, the sample in the form of a deoxygenated solution is passed

through a heat exchanger, made of a coil of viton tubing kept in a low-temperature

bath. The solution is then passed through the microwave cavity (Fig. 1a) by means

of a peristaltic pump. The low-temperature bath containing ethanol as the coolant is

cooled by pouring liquid nitrogen. The temperature of the sample in the cavity is

measured by means of a thermocouple placed inside the sample tube, but just

outside the active region of the microwave cavity. The cavity is flushed with a dry

gas.

2.2 Cooling the Sample Inside the Cavity

In this technique, a standard variable temperature dewar insert (Model WG821 of

Wilmad Glass) is used, through which pre-cooled nitrogen gas is passed. By means

of a heater and sensor assembly, the temperature of the nitrogen gas is controlled.

The liquid sample, pumped by a peristaltic pump, flows slowly through a sample

tube placed inside the cavity (Fig. 1b). This special sample tube was made of two

concentric tubes.3 The liquid sample enters the inner tube and then flows into the

outer tube, where it comes in thermal contact with the cold nitrogen gas, and then

leaves at the outlet. Controlling the temperature of the cold nitrogen gas by means of

3 The sample tube was kindly provided by Prof. S. Yamauchi, IMRAM, Tohoku University, Sendai,

Japan.

Recording of TREPR spectra using Oxygen 1009

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a temperature controller achieves the desired temperature of the sample inside the

cavity. As usual, the cavity is flushed with a dry gas.

Most of the experiments described here were done using the temperature

controlling arrangement shown in Fig. 1a. The output of the preamplifier of our

TREPR spectrometer was digitized by the transient recorder and averaged several

times to improve the signal-to-noise ratio. Figure 2a shows the observed microwave

transient signal synchronized with the laser pulse when the spectrometer was

operating in conjunction with the 266-nm laser. The cavity contained air at the

ambient temperature and no flushing gas was used. The intense sharp signal lasting

for about 0.5 ls could be seen. This was followed by a very small residual signal,

remaining almost constant during the recording time in Fig. 2a. In Sect. 1, this

slowly decaying signal was described to arise from the heating of the sample due to

the absorption of laser light. When nitrogen gas was used to flush the cavity, the

observed transient is shown in Fig. 2b. The large transient lasting for over several

hundred microseconds was easily seen. When oxygen was used to flush the cavity

(Fig. 2c), the large transient was replaced by a very small transient lasting for only

a

b

Fig. 1 Low-temperatureassembly for TREPRexperiments: a the liquid sampleis cooled externally before beingpumped into the microwavecavity of the EPR spectrometer,b the sample is cooled inside themicrowave cavity

1010 V. Rane et al.

123

about 0.2 ls. The full-width at half-maxima of this transient was about 90 ns,

compared to the value of about 200 ns that was seen when the cavity contained air.

The amplitude of the transient in the presence of oxygen was also less than half of

that in the presence of air. Very similar behavior was observed when the 248-nm

laser light was used.

The remarkable ability of oxygen gas to largely remove the long-lived transients

arising in the presence of nitrogen gas showed two important advantages. The first

is, of course, the ability to carry out the TREPR measurements at low temperatures

without overloading the preamplifier with large non-resonant signals. The other

advantage becomes apparent when one wants to record the TREPR spectra as a

function of the external magnetic field and at a fixed time after the laser pulse. Such

measurements are usually carried out by opening a boxcar gate at the desired time

and monitoring the signal while scanning the magnetic field slowly. The transient

microwave signals shows that, for such experiments, the boxcar gate could be

placed at a time much closer to the laser pulse in Fig. 2c than in Fig. 2a or b, as both

the intensity and duration of the large transient microwave signal are the smallest in

Fig. 2c. Thus, if one is interested in recording the TREPR spectrum at the shortest

possible time after the laser pulse, flushing the cavity with oxygen gas should be

employed, even if the experiment is carried out at room temperature.

To obtain some insight into the origin of the long-lived transients seen in the

presence of nitrogen gas and its disappearance in the presence of oxygen gas, we

conducted more experiments. First we noted that the origin of the long-lived

transient could not be due to ionization of nitrogen molecules by the 248- or 266-nm

laser light, as the photon energy (5.00 and 4.66 eV, respectively) is much less than

the ionization potential (15.6 eV) of nitrogen molecule [5]. Also, when the nitrogen

gas was passed through the variable temperature dewar (WG821 in Fig. 1b) and the

cavity contained air, no long-lived transient was seen. We thus hypothesized that the

interaction of the laser light with the inner walls of the microwave cavity must be

Time in microseconds

Mic

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0.5

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0.5

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0.5

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1.5

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2.5

200 400 600 8000

b

Fig. 2 Transient microwave signal recorded at the output of the preamplifier, when a 266-nm laser pulseof energy 30 mJ per pulse, operating at 30 Hz, fell in the microwave cavity. The sharp transition from thelow voltage to the maximum voltage denotes the time of incidence of the laser pulse. Note that in this andthe following figure, the zero time does not correspond to the time of the laser emission. The transientrecorder was operated in the ‘‘pre-trigger’’ mode. In this mode, the baseline microwave signal lasting for agiven duration before the laser pulse was recorded along with the ‘‘post-trigger’’ signal after the laserpulse. The low-temperature assembly was as given in Fig. 1a. The cavity was: a at an ambienttemperature and contained air, without any flushing gas; b flushed with nitrogen gas; and c flushed withoxygen gas. The displayed traces were an average of eight transients. The incident microwave power inthe cavity was 1.0 mW. The constancy of the signal at around ?2 V for certain duration of time was theresult of saturation of the preamplifier due to the strong signal

Recording of TREPR spectra using Oxygen 1011

123

producing some charged species that disturbed the microwave field inside the

cavity, and thus produced the long-lived microwave transients. The role of oxygen

was simply to quench that species.

Ours was a commonly used TE102 microwave cavity. It was made of brass, with

a silver coating that was also further coated with a gold surface. The ionization

potentials (work functions) of these materials are reported to be 4.51, 4.24,

4.56–4.73, 4.73–4.82 and 4.77 eV for copper [6], zinc [7], silver [8], gold [9] and

brass [10], respectively. Thus, the 248- or 266-nm light can easily cause

photoelectrons to be ejected from the surface of the walls of the cavity. It is

natural to expect that when the laser photon energy is less than the work function of

the metal, no ionization, and consequently no long-lived microwave transient, is

expected. We verified this expectation by doing similar experiments using the

355-nm laser (photon energy 3.5 eV) with various energies of the laser pulse while

purging the cavity with nitrogen gas. The results are summarized in Fig. 3. It shows

the transient microwave signals seen under nitrogen purging. Very small

transients were seen till the 355-nm laser energy density was increased up to about

400 mJ/cm2 (Fig. 3A). With the further increase of the laser energy density,

intensity of the transient signal increased in a highly non-linear manner, possibly by

multi-photon ionization or photo-ablation processes. This may be compared with the

typical energy density of 20–60 mJ/cm2 for the 266- or the 248-nm laser, used in

TREPR experiments and for the signals shown in Fig. 2. With these observations,

the long-lived transients could be attributed to the ionization from the walls of the

microwave cavity.

The disappearance of the long-lived transients when oxygen gas was used to flush

the cavity should be related to the ability of oxygen to remove the photoelectrons.

The probability of electron attachment to various gases has been discussed in

1

0

0.2

0.4

0.6

0.8

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0 50 100 150 200

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al, V

Time in microseconds

400

0.1

500450

Mic

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ave

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250 300 350

Laser energy (mJ/cm2)

A

B

Fig. 3 Transient microwave signals recorded at the output of the preamplifier, when a 355-nm laserpulse enters the microwave cavity, and the cavity was flushed with nitrogen gas, at two different energydensities of the laser pulse: A 300 mJ/cm2, B 500 mJ/cm2. To improve the clarity, the signal B wasdisplaced in the vertical direction by ?0.1 V. The sharp transition denotes the time of incidence of thelaser pulse. The inset shows a double logarithmic plot of the amplitude of the microwave signal at about30 ls after the laser pulse (denoted by the arrow) and the energy density of the 355-nm laser pulse

1012 V. Rane et al.

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Cobine [11], in which the electronegative character of the gases is reported to be the

sole deciding factor. The results presented there show that electrons do not attach to

pure nitrogen, hydrogen and noble gases. The gases to which electrons are

efficiently attached are air, oxygen, nitrous oxide, chlorine and water vapor. For

example, the time that electrons can remain free in oxygen at normal temperature

and pressure is reported to be about 1.4–1.9 9 10-7 s [12]. Based on these pieces of

information, we hypothesized that oxygen was not the only gas capable of removing

the long-lived transients seen in our experiments; other gases such nitrous oxide,

chlorine, and water vapor should also work equally well. As water vapor should not

be used in the microwave cavity and chlorine is hazardous, we did one test

experiment with nitrous oxide as the flushing gas. The long-lived microwave

transient disappeared in this case also, just like in case of oxygen gas, confirming its

behavior expected from the property of electron attachment.

3 Conclusion

Strong long-lived transient microwave signals are shown to arise when the pulse UV

light of wavelength 266 or 248 nm hits the sample in the microwave cavity that is

flushed with nitrogen gas in TREPR experiments. Electron ejection from the inner

walls of the microwave cavity and their non-attachment to nitrogen are proposed to

be the cause of this. When used as the flushing gas, oxygen helps reduce both the

intensity and the duration of the transient signal significantly by removing the

electrons. Thus, using oxygen as the flushing gas enables recording the TREPR

signals at low temperatures. This also allows a boxcar gate to be placed temporally

very close to the laser pulse. This enables recording of EPR spectra as a function of

the external magnetic field and at a fixed time that is very close to the laser pulse.

Warning Increasing the concentration of oxygen gas in the laboratory to a very

high value can be a potential fire hazard. As a precautionary measure, open flames

in the laboratory should be avoided when oxygen gas is used. Also, the flow of

oxygen used in these experiments should be kept at the minimum required level. We

normally use nitrogen gas to flush the cavity when in the stand by mode, and switch

to oxygen gas only during the actual recording of the TREPR spectra in the presence

of the laser light.

Acknowledgments We have greatly benefitted by discussing our results and their interpretation with

Prof. B. M. Arora. We thank him most sincerely.

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