Rotational thermometry

Both at room-temperature and 77 K, it is possible to cover virtually the com-plete R branch spectrum of the atmospheric A band of molecular oxygen. A comparison of the observed NICE-OHMS signal amplitudes thus allows to es-timate the respective rotational temperatures of the oxygen molecules, which is simply reflected in the intensity distribution of the rotational lines (Section 6.2.2). After a collective linear scaling of the experimental amplitudes, the rel-ative line strengths are indeed reproduced with good quality, as Figure 8.30 demonstrates. As expected, the rotational degrees of freedom are hence cooled

RESULTS 155

10^-20 10^-19 10^-18

SpectralLineIntensity[MHz/cm-2] ^{295 K}

760 761 762

10^-20 10^-19 10^-18

SpectralLineIntensity[MHz/cm-2]

Transition Wavelength [nm]

77.35 K

Figure 8.30: Relative line intensities at 295 K and 77 K. The bars reflect expected literature values according to the HITRAN database, as collected in Table D.1.

concurrently with the translational motion.

At 77 K, a satisfactory agreement with expected line intensities could only be obtained by applying a 65 % higher scaling factor than for 295 K. After the fit results from Figure 8.28, this is a second indication that the spectral line intensities from Table D.1 at 77 K are too high. On the other hand, this discrep-ancy might at least partially also result from a reduced detection efficiency due to a less accurate realignment of the resonator and the small area NICE-OHMS detector. A realignment is in general necessary after any substantial change in the cryostat temperature due to the thermal contraction of its dewars. Reli-able absolute measurements of absorption coefficients should therefore always be based on calibrations from the same state of alignment.

The measured transition frequencies, as derived from the individual fre-quency calibrations, show a systematic deviation of 1.52 GHz at 295 K and

156 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES 1.86 GHz at 77 K above the HITRAN values. The spread around these val-ues remains within ±200 MHz and thus below the wavemeter resolution. One reason for the systematic offset might be an inaccuracy in the experimental determination of the zero voltage emission frequency of the Ti:Sapph. This can only be measured after each NICE-OHMS trace is recorded by stopping the sinusoidal modulation of the tuning port and specifically applying a constant 0 V signal.

The spectral operating range accessible with this setup is in principle much larger than demonstrated here with Figure 8.30. It is ultimately limited by the high-reflectivity bandwidth of the cavity mirrors, which deteriorate below 720 nm and above 790 nm. In other words, the NICE-OHMS spectrometer can be used on any species with transitions in this range.

8.6.4 Studies at 4.2 K

While no effect on the cavity finesse was observed upon cooling to 77 K, its value has been reduced toF ≈13 800 at liquid helium conditions. This decrease corresponds to an increase in the round-trip loss by a factor of 7.5, which sig-nificantly deteriorates the impedance matching. By increasing the input power from the standard 1 mW to 10 mW, it has nevertheless been possible to lock the resonator to the scanning Ti:Sapphire laser under these circumstances, includ-ing NICE-OHMS modulation and FSR trackinclud-ing. This is demonstrated with the trace in Figure 8.31, which is recorded in the presence of a⁴He buffer gas at a pressure of 10⁻²mbar.

A detection of O₂ however has not been successful at this temperature, since the scanning resonator immediately became opaque after oxygen injection, most likely due to frozen oxygen on the cavity mirrors. Although it was evaporated again by warming up the cryostat, the resonator quality did not recover any-more. A subsequent microscopic inspection of the two cavity mirrors revealed

-1.0 -0.5 0.0 0.5 1.0

-3.0 -2.5 -2.0 -1.5

NICE-OHMSSignal[mV]

Detuning from 761.7336 nm [GHz]

4 K

Figure 8.31:Demonstration of NICE-OHMS scanning at 4.2 K. The small variation of the NICE-OHMS signal is similar to the features observed in Figure 8.36 and not caused by the potential presence of oxygen molecules.

RESULTS 157

200¹m 20¹m

Figure 8.32: Microscopy images of the output coupler after 4.2 K operation. A same scale image of a new mirror inspected for comparison did not show any marks.

a number of spot marks on both of them, among which also a larger circular damage was found on the output coupler (Figure 8.32). It is believed to result from the absorption of more than 1.5 W of resonating light by a film of solidified oxygen on the mirror surface. The smaller marks could be caused by a helium gas discharge fuelled by the voltages applied to the piezo, whose insulation was found damaged after warmup. An according current flow therefore should have been possible, and peak electric fields indeed must have been close to the critical value of about 87 V/mm, as estimated from the first ionization potential of ⁴He (24.5874 eV).

The total failure of the cavity at 4.2 K on the other hand is strong evidence for a successful capillary injection of gas-phase O₂ into the experimental cell, which additionally can be derived from the accompanying distinct increase in cell temperature. In contrast to the 77 K experiments, where a heating of the capillary is not necessary to prevent freezing, a current of 20 mA was applied to the Manganin heater over a period of several minutes prior to injection, whose resistance was thereby increased from its 4.2 K value to above 514.7 Ω. Accord-ing to Figure 8.12, this corresponds to a capillary temperature of approximately 49 K. At the same time, the cell temperature — as measured by the Pt-100 sen-sor — did only slightly increase from 4.22 to 4.42 K. Although the peak capillary temperature stayed below the melting point of molecular oxygen, the capillary apparently did not freeze upon injection. This is attributed to the fact that the oxygen gas stays quite warm on its way to the cell due to the intentional weak thermal anchoring of the 1/8” tubes.