Interferogram amplitude modulation technique for selective detection of transient species with a continouus-scan Fourier-transform spectrometer

1 August 1997

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 274 (1997) 99-105

Interferogram amplitude modulation technique for selective detection of transient species with a continuous-scan Fourier-transform spectrometer Takashi Imajo l, Shinobu Inui, Keiichi Tanaka, Takehiko Tanaka Department of Chemisto', Faculty of Science, Kyushu UniversiO, 33, Hakozaki, Higashi-ku, Fukuoka 812-81, Japan

Received 13 March 1997; in final form 24 April t997

Abstract

A new interferogram amplitude modulation (IAM) technique for a Fourier-transform spectrometer with a continuous-scan interferometer was devised. This technique is useful for selective detection of transient species produced in ac discharge plasmas. Infrared emission spectra due to the Av = 1 bands of the vibrationally excited CO molecule, the fundamental band of the OH radical, and the 5g-4f band of Rydberg H 2 were recorded to demonstrate selectivity of this method. © 1997 Published by Elsevier Science B.V.

1. Introduction

The technique of modulation combined with phase-sensitive detection is essential for sensitive and selective detection in spectroscopy of transient species, which are usually produced at extremely low concentration in the middle of overwhelming amount of stable molecules. In infrared laser spectroscopy, the concentration [1], velocity [2], and Zeeman [3] modulation methods have been successfully employed for highly sensitive detection of radicals and ions. The modulation technique is also desirable in Fourier transform (FT) spectroscopy to improve selectivity, especially for detection of short-lived species. However, there have been reported very few

J E-mail: [email protected].

attempts to develop modulation methods in FT spectroscopy. Martin and Guelachvili [4] have applied the velocity modulation technique to high resolution FT spectroscopic detection of the ArH + ion. Benaidar et al. [5] have recorded the FT infrared emission spectrum of the OH radical using the concentration modulation technique. In the both experiments the FT spectrometer was operated in the step-scan mode, i.e. the moving mirror of the Michelson interferometer was scanned stepwise, and the interferogram was phase-sensitively detected while the mirror stopped at each position. However, most commercially available high resolution FT spectrometers, e.g., Bruker I F S I 2 0 H R and Bomem DASPC.3X, are operated in the continuousscan mode, and the step-scan mode cannot be used unless they are specially reformed. Nafie et al. [6,7] used a continuous-scan FT spectrometer with moderate resolution to observe vibrational circular dichroism (VCD) spectra by a polar-

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1O0

72 lmajo et al./Chemical Physics Letters 274 (1997) 99-105

ization modulation method. In their experiment, polarization of the infrared source was modulated at 50 kHz by using a photoelastic modulator. The output of the infrared detector was processed by a phasesensitive detector (PSD) referenced to the 50 kHz modulation, and the desired interferogram ( = 5 kHz) corresponding to the VCD spectrum was restored from the PSD output by filtering out the highfrequency components around 50 kHz and higher. The modulation frequency (fmod) was restricted to be significantly higher than the Fourier interferogram frequencies (fF)" In this article, we describe a new interferogram amplitude modulation (IAM) method for the continuous-scan type FF spectrometer. The present IAM method is similar in principle to that of Nafie et al. [6,7]. The only difference is that the PSD output is passed through a narrow band-pass filter. This allows us to use the modulation frequency (fmod) lower than the Fourier interferogram frequency (fF)" The present modulation technique can be combined with most of commercial high resolution FT spectrometers for selective detection of short-lived species. Fourier transform infrared emission spectra from the vibrationally excited CO molecule (Av = 1), the OH radical (v = 1-0), and the 5g Rydberg state of H 2 were recorded to demonstrate selectivity for detection of transient species.

I Modulated .

2. Principle Fig. 1 schematically shows the principle of the present IAM method. For simplicity, suppose that we observe a monochromatic emission source at the wave number tr by using a Fourier transform spectrometer with a continuously scanned interferometer, whose moving mirror is displaced at the velocity V. If the emission source is not modulated, the interferogram observed by a detector is given by I -~ [1 + COS(2"n'fFt)],

(1)

where fF = 2Vo- is the Fourier interferogram frequency and I denotes the emission intensity. When discharge modulation at the frequency fmod is applied to the emission source, the time dependence of the emission intensity may be represented by l( t) = I o + llsin(27rfmodt + y ) + 12sin(4~fmodt + Y2) + . . . .

(2)

where I 0 is the average intensity of the emission, lj corresponds to the modulated component at the fundamental frequency and 12 ... to the harmonic components. For the moment we consider an ideal case in which the harmonic components may be ne-

fmo~

IR Source

I 0 +11 sin(2~fm~ t+7)

~sin(2~fmodt+~)

B 4"'''

Ref Fixed /

Mirror ~

.

- ~

.

\,

I

locos(2~fv t ) + 11COS(2~fFt ) sin(2fffmod t +]')

÷o,.

~D-~ Without time c o n s t a n t

P

~pF~] I~ ,I~ +-I ~ ,""

Moving Mirror

Fig. 1. Principle of the interferogram amplitude modulation (IAM) method. The modulated intefferogram is processed by a phase-sensitive detector (PSD) at fmoa" The PSD output is passed through a narrow band-pass filter (BPF), which passes the frequencies around fv and blocks unwanted frequency components, fF +fmod . . . . . The former is fed to the A / D converter and subjected to Fourier transformation.

72 lmajo et al. / Chemical Physics Letters 274 (1997) 99-105

glected. Then, replacing 1 in formula (1) by l(t), we obtain the interferogram signal ½cos(27rfvt) [ Io + llsin(2~fmo J + y ) ] + ½[10 + 1,sin(27rfmodt + Y)],

(3)

which is fed to a phase-sensitive detector (PSD) with zero time constant. As well known [8], demodulation by the PSD at fmod is equivalent to multiplying by the reference signal sin(27rf,lod t + 6 ) ,

(4)

where 6 is an adjustable phase in phase-sensitive detection. The resulting PSD output is ½sin(Zrr.)%odt + 6 )cos(Zrrfzt ) × [10 + I, sin(Z~-fmodt + Y)] + ½sin(27r./n, oat+ ~)[Io + I, sin(2rrfmoat + Y)].

(5) The first line of formula (5) gives rise to a component with the frequency fv, Ii --cos( a - 7)cos(2~-.fF t ) . 4

The demodulated interferogram in formula (6) has a phase factor of c o s ( 6 - y), therefore the phase shifter of the PSD must be optimized for the maximum output. The optimum phase contains information on formation and destruction of the observed species. If the emission source is insensitive to the discharge modulation (I1 = 0), no output will be observed, allowing us to selectively detect transient species.

3. Experimental Fig. 2 shows the emission cell used for the present study, which consists of a 30 cm long, 8 mm internal-diameter pyrex tube, and two discharge electrodes in water-cooled side arms. The both ends of the cell were sealed off with calcium fluoride windows. The sample gas was supplied from two inlets near the ends of the pyrex tube and the buffer gas was introduced through the side arms. The emission cell was evacuated by a roots pump (240 m3/h) directly connected to the center of the pyrex tube.

(6)

Formula (5) also has components with frequencies

)¢v +Jmod" .)% --fmoa, ./'F + 2fmod' fF -- 2fmod, fmod, and 2fmoa. Furthermore, the harmonic components of the modulation can give rise to components at frequencies fF + nfm,,a, .fF -- nfmod, and nfmoa, with n greater than 2. The PSD output is passed through a band-pass filter, which eliminates components other than that of formula (6). Since formula (6) is proportional to the ac component of the original interferogram given in formula (1), the spectrum in the wave number domain can be restored from it by an ordinary process of Fourier transformation. The bandwidth W (FWHM) of the band pass filter should be chosen considerably narrower than .f,~,od, because unwanted frequency components at fv +fmoJ and f F - f m o J nmst be suppressed. As a result the range that can be observed, Au, in the wave number domain is restricted to W A u = 2--T'

101

(7)

where V is the scan velocity of the moving mirror.

3k~

',

30:1

Ar

..Z

1 -%.

l

i

BrukerIFS120HR I

P gasin

1 240m 3 / h

Fig. 2. Schematic diagram of the discharge emission cell. Ac high voltage generated by an audio power amplifier and a step-up transformer is applied to water-cooled electrodes in side arms. Infrared emission is focused by a CaF2 lens onto the input aperture of the FT spectrometer.

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Ac high voltage of 1-60 kHz generated by an audio power amplifier (Accuphase P-600, 1 kW) and a step-up transformer (1:30) was applied between the electrodes through a rectifying diode and a ballast resistor (3 kl)), and the emission was modulated at fmod = 1-60 kHz. The peak current of the discharge was typically 500 mA. When the rectifying diode was removed from the circuit, discharge occurred twice, in alternative directions, in a cycle of the power supply, so that the emission was modulated at fmod = 2-120 kHz. The emission cell was placed in front of the emission port of a FT spectrometer (Bruker IFS 120HR) with a narrow air gap of about 1 cm. A CaF 2 lens ( f = 500 mm) focused infrared emission from the discharge tube onto the entrance aperture. A CaF 2 beam splitter was used in the Michelson interferometer. Infrared power was monitored by a liquid N 2 cooled InSb detector through an appropriate optical band-pass filter (Spectrogon). As a typical example, the experimental condition for the measurement of the OH emission was as follows. The velocity of the moving mirror was 1.9 cm s - I , which corresponded to the Fourier interferogram frequency fF = 13.6 kHz, when the emission occurred at 3300 cm -1. The modulation frequency fmod was typically 2 kHz. The output of the PSD with zero time constant was passed through a narrow band-pass filter centered at 13.6 kHz with a width of 0.5 kHz FWHM, which eliminated unwanted frequency components at 11.6, 15.6 kHz, etc. The resolution of the FT spectrometer was 0.05 cm 1. The spectrum was recorded by accumulating 160 interferometer scans, taking a total time of 50 min. In the following section, the spectrum recorded without phase-sensitive detection is compared with those observed by the IAM method. The former is obtained using completely the same experimental setup except that the reference signal to the PSD is switched off. Then, the PSD output corresponds to formula (3), because the PSD works as a simple amplifier. The component

I0

-~- c o s ( 2 7rfF t )

4. Observed spectra 4.1. Vibrationally excited CO As an initial test of the present IAM technique, we measured infrared emission spectrum from the vibrationally excited CO molecule. The measurement was performed using ac discharge (fmod = 2 kHz) in the flowing gas mixture of CO (1 Pa) and Ar (10 Pa). The peak discharge current was 210 mA. Fig. 3 is a part of the spectrum, showing the v -~ v - 1 rovibrational transitions from the v = 8, 7, 6, and 5 upper states, as assigned according to Ref. [9]. The asterisked line is due to Rydberg Ar. Weak lines for which assignments are not explicitly indicated belong to the v = 9 - 8 and 4 - 3 bands. Trace (a) was recorded without phase-sensitive detection. In comparison, traces (b), (c), and (d) were recorded by the IAM technique, with phase differences A 4) of 0 °, 45 °, and 90 °, respectively, between the discharge current and the reference signal for phase-sensitive detection. In trace (b) ( A4~ = 0°), only the Ar emission line was observed and the CO lines almost vanished. On the contrary, in trace (d) (A~b= 90 °) no Ar line was seen but the CO lines

5

4

I

8 I

~"

n<

2 i

v=5-4 PIJ")

15 I

4

remaining after filtering is subjected to Fourier transformation and gives the spectrum corresponding to the average intensity ( I 0) of the emission.

9 I

v=6-5 P(J")

3 i 16 I

3 2 0

1 0 i

i

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I

i

1980

i

,

I

I

1975 Wavenumber

(8)

v=8-7 R(J")

I

v.--7-6 P~J")

/ cm -1

Fig. 3. Infrared emission spectrum from vibrationally excited CO. Trace (a) was recorded without phase-sensitive detection. Traces (b), (c), and (d) were recorded by the IAM technique (fmod = 2 kHz), with phase differences of 0 °, 45 °, and 90 °, respectively, between the discharge current and the reference signal of the PSD. The asterisked line is due to Rydberg Ar.

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P(4.5)

e t al. / C h e m i c a l

Physics

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:ir"

5

~4 ~

3

~

2 ¸

(c)

i

i

i

i

i

•

i

. . . .

3380

i

. . . .

i

,

,

3370 Wavenumber

/ cm

-1

Fig. 4. Infrared emission s p e c t r u m for the f u n d a m e n t a l b a n d of O H . Trace (a) was recorded without phase-sensitive detection. W e a k e r lines are H 2 0 transitions: n , o, and * represent v], u 3, and u] + u 3 ~ u]. Traces (b) and (c) were m e a s u r e d b y the I A M method with Jmoa = 2 and 20 kHz, respectively.

were observed most strongly. These observations indicate high selectivity of the present IAM method. The optimum phases indicate that the Ar emission occurred with no delay from the discharge current but that the peak of the CO emission was delayed by about 80 p,s. The vibrational relaxation is an important process to produce vibrationally excited CO. The lifetime of Rydberg Ar is less than 1 txs, whereas that of vibrationally excited CO is estimated to be a few ms in the present condition [10]. In this situation, the modulation frequency fmod = 2 kHz is appropriate to fully modulate the concentration of At, but too high for vibrationally excited CO. The inadequately high fmod might result in the loss of the S / N ratio in trace (d) compared with trace (a).

Letters 274 (1997)

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99-105

Traces (b) and (c) were recorded by the IAM technique using discharge modulation at fmod = 2 kHz and 20 kHz, respectively. When fmod = 2 kHz, the OH spectrum had almost the same intensity as in trace (a). Water transitions in the u 1 and u 3 bands are suppressed in trace (b) although the transition in the u] + u 3 - u I band remains. The OH signals disappeared entirely at fmod = 20 kHz, probably because the lifetime of the OH radical, estimated to be about 0.5 ms, is too long to be modulated at 20 kHz. The optimized phase in trace (b) was 0 °, implying that the generation of the OH radical was delayed very little from the discharge current. 4.3. 5g Rydberg H,

The lifetime of Rydberg H 2 generated in a discharge plasma has been measured to be less than l0 p,s [14], which is much shorter than those of the OH radical and vibrationally excited CO. Davies et al. [15] reported the infrared absorption spectrum of the ~3 + a-~g --~31-lu band observed by tunable diode laser spectroscopy using concentration modulation at 100 kHz. In the present study, the emission spectrum for the Rydberg 5 g - 4 f band was observed by the IAM technique. Trace (a) in Fig. 5 shows the spectrum

R 1(2)

•,,

4

R1(2)

3

R1(4) Iv=l)

2

v°3, I

R1(4) (v=l)

(v=0)

I

R1(4)

(v=0)

I I

I

Ro(3)

t i

I

v°o)

0

4.2. OH radical o _c

Infrared emission due to the fundamental band of OH )~ 2H was measured by the IAM method (Fig. 4). Trace (a) was recorded without phase-sensitive detection, using ac discharge (500 mA) in the gas mixture of H 2 0 (5 Pa) and He (10 Pa). The rovibrational assignment was made according to a previous infrared study [11]. Signals marked with tB, o, and * are water transitions in the ul, u 3, and u] + u 3 u] bands [12,13].

3

1

)

0

10

2500 Wavenumber

90

2480 / cm-1

Fig. 5. Infrared emission spectrum of the 5 g - 4 f b a n d of R y d b e r g H 2. Trace (a) w a s recorded without phase-sensitive detection. Trace (b) was recorded by the I A M method using fmoa = 60 kHz.

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T. lmajo et al. / Chemical Physics Letters 274 (1997) 99-105

recorded without phase-sensitive detection. Trace (b) was observed by the IAM method using rectified discharge through pure H 2 gas (200 Pa) with a peak current of 500 mA at a frequency of 60 kHz. The assignment was straightforward from a reported FTIR emission spectrum [16]. Weak unassigned lines in traces (a) and (b) are also transitions of Rydberg 5 g - 4 f band. The S / N ratio in trace (b) is slightly better than in trace (a). The IAM technique using high frequency modulation is effectively applicable to Rydberg molecules and ions whose lifetimes are typically shorter than 100 Vs.

5. Discussion The interferogram amplitude modulation (IAM) technique was successfully applied to a continuousscan type FT spectrometer. Emission spectra from the vibrationally excited CO molecule, the OH radical, and the Rydberg state of H 2 were observed to demonstrate selectivity of the IAM method. The CO spectrum was selectively detected by optimizing the phase angle of the PSD, discriminated from the Ar spectrum. The spectrum of the OH radical was discriminated from most H 2 0 lines by discharge modulation at fmod = 2 kHz, but fmoa = 20 kHz was too high for this radical. The spectrum of the Rydberg H 2 was observed using a very high modulation frequency of 60 kHz. In the previous experiment by Nafie et al. [6,7], they were forced to use the modulation frequency much higher than that of the interferogram, because a low-pass filter was used to separate the demodulated interferogram from the unwanted frequency components. In the present system with a narrow band-pass filter, the modulation frequency in the range from 1 to 100 kHz can be selected so as to match the lifetime of the transient species of interest. A relatively low modulation frequency, e.g., 1 kHz, is suitable for radicals and relatively high one, e.g., 100 kHz, for molecular ions. A drawback of the present IAM method is that the wave number region observable in a single experimental run is limited by the bandwidth of the bandpass filter [Eq. (7)]. In the case of the OH spectrum, the 0.5 kHz FWHM band-pass filter centered at 13.6 kHz corresponds to the observable wave number

range with a full width of 120 cm ~ centered at 3300 cm -1, the scan velocity of the moving mirror being V = 1.9 cm s -m. It seems that the Fellgett's advantage [17] (wide observable range at once) of FI" spectroscopy is sacrificed. However, this disadvantage will not be too serious, especially in highresolution measurements. It is also noted that demodulation procedure may give rise to spurious signals in the observed wave number region, if there occur emission lines at wave numbers displaced by about fmoJ2V. These emission lines must be eliminated by using an optical band-pass filter. In the case of OH, emission lines around 2800 and 3800 cm -1 may create spurious signals, however, they were easily cut by an optical filter with a width of 500 cm-1. Finally, the present 1AM technique should be applicable with little modification to FT absorption spectroscopy combined with concentration modulation.

Acknowledgements The authors are thankful to Dr. K. Harada at Kyushu University for helpful discussions. Financial support by the Ministry of Education, Science, Sports and Culture is acknowledged.

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[11] J.P. Maillard, J. Chauville, A.W. Mantz, J. Mol. Spectrosc. 63 (1976) 120. [12] A.S. Pine, M.J. Coulambe, C. Camy-Peyret, J.M. Flaud, J. Phys. Chem. Ref. Data 12 (1983) 413. [13] C. Camy-Peyret, J.M. Flaud, G. Guelachvili, C. Amiot, Mol. Phys. 26 (1973) 825.

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[14] J.A. Sanchez, J. Campos, J. Phys France 49 (1988) 445. [15] P.B. Davies, M.A. Guest, S.A. Johnson, J. Chem. Phys. 88 (1988) 2884. [16] G. Herzberg, Ch. Jungen, J. Chem. Phys. 77 (1982) 5876. [17] P.B. Fellgett, J. Phys. Radium 19 (1958) 187.