Possible Detection Methods

In the pulsed jet operational mode, the absorption line can be recorded by either a 2f phase sensitive detection method using lock-in amplifier or by a gated detection method using a boxcar gate integrator triggered by the jet expansion repetition rate which has a maximum frequency of 100 Hz. This is the so-called jet modulation technique or "concentration modulation". The sensitivity of this technique is limited by the 1/f low frequency excess noise of the TDL caused mainly by mechanical vibrations from closed cycle helium cooler (cryostat), compressor, pumps and other sources. The effect of this laser noise will be more pronounced and significant when the lock-in amplifier triggered by the jet frequencies (~ 100 Hz) is used. The laser excess noise can be noticeably reduced by using high repetition pulsed valve producing short duration pulses of few 100 µs. However, these frequencies are still too small to overcome the detector-preamplifier thermal noise. For this reason, two different detection methods have been established, using a combination of pulsed slit- nozzle and tunable diode laser spectrometer, which enables the detection of absorption signals as small as 10-4 - 10-5 at frequencies of few 10 kHz. These methods will be introduced in the following

two sections.

4.3.1 The Rapid-Scan Method

This method is commonly used in slit nozzle experiments with low repetition rates of few hertz and pulse duration in the millisecond range. In this method, a selected wavelength range (one laser mode) is measured and recorded over the pulse duration associated with a rapid scan of the laser current. In addition to this open valve measurement, a second measurement is also recorded with closed valve. Both scans are then subtracted from each other to scale down the background noise. A high signal to noise ratio can be achieved if several 100 to several 1000 of the pulses are averaged

and added together. De Piante et al demonstrated the use of the rapid-scan method on the spectra of the Ar-CO-complex (108), Sharpe et al (118, 119) on HX-CO2 and Ar-CO2

and Dutton et al (120) on CO2-N2O. Hu et al (110) observed the Van der Waals complex

N2O-Ar using this technique, whilst Brooks et al and Xia et al used this method to

spectroscopically investigate (CO)2,CO-H2O and CO-N2 (113-116) . The most serious

problem of this method is the temperature drift of the laser frequency. This means that each individual scan begins at a slightly different frequency, which inevitably leads to a broadening of the spectral lines as soon as the scans are averaged. To overcome this problem, Hu et al developed a fast electronics to stabilize the drift in the laser frequency. The fast electronics work as a feedback circuit to add the change in the etalon signal due the thermal drift to the laser current as an error signal. Hu et al developed this laser electronics because the widening of spectral lines started to be more and more significant after averaging over 1000 scans at a repetition rate of 3 Hz. This type of averaging needs an exceptional temperature controller cryostat with large cooling power and a rapid heating system. Such disadvantage does not apply in our experimental setup because the laser needs a few seconds to return from the end to the beginning of a mode, during which the temperature stabilizes itself.However, this fact would limit the highest possible repetition rate significantly. Another disadvantage of this method is that it only functions with the diodes with single-mode operation, whereas it is impossible to scan the monochromator over several wave numbers within a millisecond range. This method also shows more laser intensity fluctuation in the base line of the spectrum, which makes the use of at least a 12 bit analog-digital- converter essential in order to achieve the required resolution of the absorption lines. These disadvantages can be eradicated using the so-called step-scan method, which was employed for the nozzles used in this work.

4.3.2 The Step-Scan Method

The step-scan procedure is based on increasing the laser current in single steps, whereas many measurements are taken at every opening and closing time of the nozzles. The modulation of the laser frequency is kept the same as used in the continuous slit nozzle which was mentioned in section 4.1. The use of modulation frequencies in the region of a few 10 kHz effectively reduces the 1/f-laser noise. The nozzle was operated with opening durations of 2-3 ms and repetition rates of 40 Hz. If

the laser frequency is tuned to an absorption line, then the detector signal consists of two components in addition to the laser modulation frequency, which corresponds to the addition and difference of the laser modulation and pulse frequencies. A two-step demodulation process is then required in order to extract the signal components. The process starts by multiplying the input signal of the lock-in amplifier with the reference frequency produced by the lock-in amplifier. The resultant signals constitute the addition and the difference frequencies of the signal components along with the reference frequency. For example, if the laser modulation frequency lies at 10 kHz and the pulse frequency at 100 Hz, then the signal in front of the lock-in amplifier contains frequency components at 9.9, 10 and 10.1 kHz. While in 2f technique the multiplication produces frequencies of 0, 0.1, 19.9, 20 and 20.1 kHz. In this case, since the acquired signal is contained in the 100 Hz components, the lock-in amplifier generates the relevant signal (different from the usual DC components) to work as a band pass filter. In principle, the lock-in amplifiers direct the multiplied signal to a low pass filter, which normally damps all AC-components above the threshold value of some cut-off frequencies in the signal and pass only the lower DC frequency components. But since this is not desired here, the bandwidth of the low pass filter which increases with decreasing time constant of the lock-in amplifier, must be set high enough so that the signal components of the pulse frequency (in the above example the component at 100 Hz) can pass through the low pass unaltered. This is the case for time constants smaller than 1ms. In the case of the gated detection method using boxcar-integrators, one can integrate over two time intervals one before the pulse and one during the pulse and then average over a certain number of pulses at each laser frequency. A second lock-in amplifier, which operates with the pulse frequency as reference frequency, can be used instead of the Boxcar-integrators. This method was used initially by Sharpe et al in the investigation of the CO2-Ar complex (107)

. Quian et al also used it in conjunction with a pulsed point-nozzle to investigate the (N2O) 2 (121) and N2O-noble gas complexes (122), whereas Pak et al demonstrated

this technique on Ar-CO (123) and used it to study CH4-Ar and CH2-Kr complexes (117).

Qian et al employed both rapid-scan and step-scan methods to investigate N2O-CO

complexes (124). The above discussion showed that the step-scan procedure leads to a better resolution as well as a higherproductivity of the spectrometer.