JP2879995B2 - Depth analysis using photoacoustic and Fourier spectroscopy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to photoacoustic spectroscopy (PA).
S) and a depth analysis method of a sample using Fourier spectroscopy, and in particular, PAS and Fourier which can accurately detect information in the depth direction of the sample using a pulsed light source and a rapid scan interferometer. The present invention relates to a depth analysis method using spectroscopy.

[0002]

2. Description of the Related Art Photoacoustic spectroscopy (PAS) is as follows. That is, the sample is irradiated with light, a part of the light energy captured therein generates heat, and a part of the generated heat energy is transmitted to the surrounding gas by thermal diffusion. Now, if the irradiation light is intermittently modulated and has a sufficiently high heat conversion rate compared to its modulation frequency, the transfer of heat energy to the surrounding gas will also be intermittent,
A compression wave (sound wave) is generated in the surrounding gas. This can be detected by a high-sensitivity microphone to obtain spectral information of the sample. As the above-mentioned intermittently modulated irradiation light, light modulated by a rapid scan interferometer is used, and a signal detected by a microphone is processed using the same signal processing as FT-IR (Fourier transform infrared spectroscopy). By changing the moving mirror speed of the interferometer, information in the depth direction of the sample is obtained (Mitsuo Tasumi, "Basics and Practice of FT-IR" (1986)
December 10th, 2002 (published by Tokyo Chemical Co., Ltd.) 114-1
See page 23).

By the way, the present applicant has already generated a stimulus periodically by the stimulus generation means and repeatedly applied the stimulus to the object to be measured. The output of the detector using the rapid scan interferometer is the interferometer for a certain delay time from the stimulus. The time-resolved spectroscopy method of measuring a reaction state of a measurement object that repeatedly shows the same response to a stimulus by acquiring a gram and obtaining a spectrum by Fourier transform is disclosed in Japanese Patent Application Nos. Hei. No. 335748, Japanese Patent Application No. 2-259355, Japanese Patent Application No. 2-259357,
Japanese Patent Application Nos. 3-2378, 3-2379, 3-64623, 3-64624, and 3
Various proposals have been made, for example, in US Pat.

[0004]

The above-mentioned FT-IR
In the PAS using, the information in the depth direction is the average absorption information from a certain depth to the surface of the sample, and since the depth also varies depending on the wave number of light, information on only a layer at a certain depth is obtained. It is difficult to take out.

In order to analyze only the surface of the sample, the speed of the movable mirror may be increased. However, it is difficult to actually increase the speed of the movable mirror due to the conversion speed of the AD converter.

The present invention has been made in view of such circumstances, and has as its object to use photoacoustic spectroscopy and Fourier spectroscopy which can accurately detect information in the depth direction of a sample. It is to provide a depth analysis method.

[0007]

The depth analysis using photoacoustic spectroscopy and Fourier spectroscopy according to the present invention, which achieves the above object, comprises irradiating a sample to be measured with light modulated by a rapid scan interferometer. The heat generated in the measurement sample is diffused and transmitted to the surrounding gas, and the change in the density of the gas based on the diffusion is detected and converted into an electric signal. In depth analysis using photoacoustic spectroscopy and Fourier spectroscopy to acquire information, using a pulsed light source that emits light at a predetermined period as a light source, sampling a detection signal at a fixed delay time after the pulsed light source emission, A frequency component in a predetermined range of the sampled signal is extracted, Fourier-transformed, and heat of a specific depth corresponding to the delay time of the measurement sample depending on the irradiation light wave number is obtained. A method characterized by obtaining fabric information.

[0008]

According to the present invention, a pulse light source emitting light at a predetermined period is used as a light source, a detection signal is sampled at a fixed delay time after the light emission of the pulse light source, and a frequency component in a predetermined range of the sampled signal is extracted. Since Fourier transform is performed to obtain heat distribution information at a specific depth corresponding to the delay time of the measurement sample depending on the wave number of the irradiation light, absorption in the depth direction of the measurement sample depending on the wave number of the irradiation light By utilizing the difference in characteristics, information in the depth direction of the sample can be accurately detected.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, an embodiment of a depth analysis method using photoacoustic spectroscopy and Fourier spectroscopy of the present invention will be described below. FIG. 1 shows a basic configuration of an apparatus for carrying out one embodiment of a depth analysis method according to the present invention. In this embodiment, the modulation frequency f of the interferometer 2 is
Is present only in a range smaller than half of the repetition frequency 1 / τ (τ: pulse emission repetition cycle). FIG.
, 1 is a pulse light source, 2 is an interferometer, 3 is a timer for causing the pulse light source 1 to emit a pulse with a period τ,
Reference numeral 4 denotes a sample from which information in the depth direction is to be obtained, and reference numeral 5 denotes a microphone for detecting sound waves due to heat from the sample 4, and the sample 4 and the microphone 5 are PA
The S unit 6 is constituted. Further, 7 is a preamplifier, 8 is a delay circuit, 9 is a gate circuit, 10 is a low-pass filter, 11 is a main amplifier, 12 is an AD converter, 13
Indicates a computer.

In such a configuration, the sample 4 is uniformly irradiated with the light from the pulse light source 1 modulated by the interferometer 2, and absorbs light corresponding to the component of the layer at each depth. The absorbed light is converted into heat and exhibits a heat distribution according to the absorption. The heat spreads over time and reaches the surface of the sample 4, where it warms and expands the surrounding air. The microphone 5 catches the pressure change as sound and converts it into an electric signal.
The timer 3 causes the pulse light source 1 to emit a pulse at a constant period τ asynchronously with a reference signal of the interferometer 2 (sampling signal for A / D converting an interferogram and taking it into a computer and performing Fourier transform). Further, the delay circuit 5 is provided with a predetermined time Δ
The gate circuit 9 is controlled by generating a trigger delayed by τ. The gate circuit 9 cuts out the output from the microphone 5 at a time delayed by a predetermined time Δτ from the pulse emission, and has a gate width sufficiently narrow with respect to the period τ similarly to the AD converter 12. , A comb-like signal is obtained by passing the signal only during that time.
The low-pass filter 10 extracts a low-frequency component (f <1 / 2τ) of the spectrum obtained from the output of the gate circuit 9.

When a signal (pulse response function of PAS) obtained from the microphone 5 when the sample 4 is irradiated with pulse light having a unit intensity of the wave number σ of light is A (σ, t), A (σ, t) t) represents the heat distribution information of the sample 4 in the depth direction with respect to the light having the wave number σ. Therefore, this PA
By obtaining the pulse response function A (σ, t) of S, information on the sample 4 in the depth direction can be accurately detected.

Now, the pulse train Шτ (t) of the monochromatic light B (σ) dσ
Is expressed as B (σ) dσШτ (t) cos2πx σ. If this is incident on the PAS unit 6, the signal obtained from the microphone 5 is expressed as ｛B (σ) dσШτ (t) cos2πx σ｝ * A (σ, t) = ｛A (σ, t) * Шτ (t)｝ B (σ) cos2πxσdσ Here, Шτ (t) is a comb function representing a repetitive operation in which the Delak function δ (t) is arranged at equal intervals of time τ, *
Is a convolution operation. Therefore, when light of various wave numbers is emitted from the pulse light source 1, the signal obtained from the microphone 5 is as follows.

∫ ｛A (σ, t) * Шτ (t)｝ B (σ) cos 2πx σ dσ When this signal is cut out through the gate circuit 9, Шτ (t−Δτ) ∫ ｛A (σ, t ) * Шτ (t)｝ B (σ) cos2 πx σ dσ

In this signal, Шτ (t−Δτ) indicating the sampling process is a function independent of the variable σ, and therefore can be incorporated in the integration. Here, only {τ (t−Δτ)} A (σ, t) * {τ (t)} dependent on the time variable t is extracted, and the equation is modified. Since the comb function can be represented by the sum of the delta functions, Шτ (t) = Σδ (t-nτ) ... (A1) Шτ (t-Δτ) ｛A (σ, t) * Шτ (t) こ のA1)
Rewrite using the formula. Here, n is an integer.

Шτ (t-Δτ) ｛A (σ, t) * Шτ (t) ｛Σ = ｛Σδ (t-Δτ-nτ)｝ [∫A (σ, t-t ') ｛Σδ (t'- mτ)｝ dt '] = ｛Σδ (t-Δτ-nτ)｝ ｛ΣA (σ, t-mτ)｝ (A2) m is also an integer. Since one cycle τ of the pulsed light source is longer than the thermal diffusion time of the sample 4, when t <0, t ≧ τ, A (σ, t) = 0. Therefore, in the equation (A2), a signal is obtained only when n = m. Therefore, (A2)
The equation can be modified as follows.

｛Σδ (t−Δτ−nτ)｝ ｛ΣA (σ, t−mτ) Σ ｛= Σ ｛δ (t−Δτ−nτ) A (σ, t−nτ)｝ = Σ ｛δ (t -Δτ-nτ) A (σ, Δτ) ・ ・ ・ (A3) Since A (σ, Δτ) is a constant and can be rounded down, = A (σ, Δτ) Σδ (t- Δτ-nτ) = A (σ, Δτ) Шτ (t- Δτ) ・ ・ ・ (A4) Therefore, Шτ (t-Δτ) ∫ ｛A (σ, t) * Шτ (t)｝ B (σ) cos2 πx σ dσ = Шτ ( t-Δτ) ∫ A (σ, Δτ) B (σ) cos2 πx σ dσ (A5)

When the signal (A5) obtained through the gate circuit 9 is closely looked at, the applicant of the present invention has filed Japanese Patent Application No. 1-2302.
No. 09, Japanese Patent Application No. 1-335748, Japanese Patent Application No. 2-259
No. 355, Japanese Patent Application No. 2-259357, Japanese Patent Application No. 3-23
No. 78, Japanese Patent Application No. 3-2379, Japanese Patent Application No. 3-64623
No., Japanese Patent Application No. 3-64624, Japanese Patent Application No. 3-158330.
The signal (A5) is of the same type as the output from the gate circuit in the time-resolved spectroscopy method proposed in the Japanese Patent Application No. 3-2378. . That is, the light that irradiates the sample 4 from the pulse light source 1 through the interferometer 2 corresponds to the repetitive stimulus given to the measurement target from the stimulus generation means in the above-described time-resolved spectroscopic method, and the subsequent signal processing is performed in exactly the same manner. That is all we need to do.

Therefore, in the case of FIG. 1, since the modulation frequency f of each time-resolved signal of the interferometer 2 exists only in a range smaller than half of the repetition frequency 1 / τ of the pulse light source 1, Japanese Patent Application No. 1-230209 is disclosed. Signal, the low frequency component (f <1 / 2τ) of the signal is
After amplification by the main amplifier 11, sampling is performed by the AD converter 12 using the reference signal having the period τ ₀ of the interferometer 2, and a Fourier transform is performed by the computer 13 to obtain A (σ, Δτ) B (σ) Is obtained, for example, using B (σ) obtained by ordinary FT-IR spectrometry,
By taking a ratio with the obtained A (σ, Δτ) B (σ), the PAS pulse response function A (σ, Δτ) representing the heat distribution information in the depth direction with respect to the light of the wave number σ of the sample 4 is obtained. You can ask. Similarly, by changing the delay time Δτ of the trigger signal to the gate circuit 9, the pulse response function of the time-series PAS in a series of delay times is obtained.
A (σ, Δτ) can be obtained (Δτ corresponds to the depth).

The above is a case where the modulation frequency f of the interferometer 2 exists only in a range smaller than half the repetition frequency 1 / τ (τ: pulse emission repetition cycle) of the pulse light source 1, but m / 2τ If the spectrum from the pulse light source 1 exists only in the range of <f <(m + 1) / 2τ (m is a positive integer), the signal obtained from the gate circuit 9 is described in Japanese Patent Application No. Hei. Processing is performed in the same manner as in Japanese Patent Application Nos. 3-2379 and 3-158330. That is, in the case of Japanese Patent Application No. Hei 3-2379, the high-order component (the first-order component within the range of f <1 / 2τ) of the signal obtained from the gate circuit 9 by the low-pass filter 10 in the apparatus shown in FIG. ) And outputs the signal to the AD converter 12
, The phase correction and the wave number conversion are performed by the computer 13, and the pulse response function A (σ,
Δτ).

In the case of Japanese Patent Application No. 3-2378, as shown in FIG. 2, instead of a low-pass filter,
Using a band-pass filter 14 that passes a signal of m / 2τ <f <(m + 1) / 2τ (m is a positive integer), the signal obtained from the gate circuit 9 corresponds to the original PAS response function. Only the part (0-order component) is taken out and Fourier-transformed as in the case of FIG. 1 to obtain a pulse response function A (σ, Δτ) of the PAS.

Further, in the case of Japanese Patent Application No. 3-158330, similarly to the case of Japanese Patent Application No. Hei 3-2379, the high-order component (f <1 / 2τ) of the signal obtained from the gate circuit 9 by the low-pass filter 10 is used. , But instead of performing phase correction and wave number conversion, a signal from the low-pass filter 10 is input to the lock-in amplifier 15 as shown in FIG. 1 is converted into a signal having the same spectrum as the original PAS response function and requiring no phase correction, and is sampled by the AD converter 12.
As in the case of (4), the pulse response function A (σ, Δτ) of the PAS is obtained by Fourier transform.

1 and 3, the pulse light source 1
The light emission cycle is τ, the cycle of cutting out the signal by the gate circuit 9 is τ / 2, every other signal is a PAS signal when pulsed light is incident, and the other signal is not pulsed light. By using the background signal at the time of (1), it is possible to perform measurement that is not affected by changes in the state of the device and the measurement environment. That is, Japanese Patent Application No. 1-3
In the case of 35748, the modulation frequency f of each time-resolved signal of the interferometer 2 is equal to the repetition frequency 1 / τ of the pulse light source 1.
Is present only in a range smaller than half of the pulse light source 1 as shown in FIG.
And the other signal is a multiplier 1
The signal is converted into a signal having a period τ / 2 by 6, and then a trigger delayed by a predetermined time Δτ is generated by the delay circuit 5 and applied to the gate circuit 9. The gate circuit 9 cuts out the output from the microphone 5 at a time delayed by a predetermined time Δτ from the time point of the pulse light emission and the intermediate point between the pulse light emission. The frequency component of 1 / 2τ <f <1 / τ of the signal from the gate circuit 9 is extracted by the band-pass filter 14, and the signal from the band-pass filter 14 is input to the lock-in amplifier 15. By synchronizing with the signal of τ, the phase is not required to be corrected. After sampling by the AD converter 12, the computer 13 performs Fourier transform to obtain A in the form of a difference from the background.
(σ, Δτ) B (σ) is obtained, and using the separately obtained B (σ), the pulse response function A (σ, Δτ) can be obtained. Similarly, by changing the delay time Δτ of the trigger signal to the gate circuit 9, the pulse response function A (σ, Δτ) of a time-series PAS at a series of delay times can be obtained (Δτ is the depth. Corresponding.)

In the case of Japanese Patent Application No. 3-64623, the modulation frequency f of each time-resolved signal of the interferometer 2 is m / 2.
This is a case where there exists only in the range of τ <f <(m + 1) / 2τ (m is a positive integer). In this case, instead of using the band-pass filter 14 of FIG. 4, as shown in FIG. 1
A low-pass filter 10 for extracting components in the range of / 2τ is used. Others are the same as those in FIG.

Further, according to Japanese Patent Application No. 3-64624,
The lock-in amplifier 15 shown in FIG. 5 is omitted and the configuration shown in FIG.
After sampling at 2, the computer 13 performs phase correction and wave number conversion to obtain a difference from the background.
The pulse response function A (σ, Δτ) of the AS may be determined.

In each of the above-described FIGS. 1 to 6, a plurality of channels including a gate circuit 9, a low-pass filter 10, a main amplifier 11, etc. are provided, and a signal from the preamplifier 7 is input to each channel. By making the delay time Δτ from when the light source 1 emits light to when each gate is opened different for each channel, the PAS pulse response function A () in a series of different delay times Δτ ₁ , Δτ ₂ , Δτ _3. σ, Δτ) can be obtained at the same time. The example is shown only in FIGS. 7 and 8 for the case of FIG. 1 and the case of FIG. 4, but FIG. 2, FIG. 3, FIG.
It will be apparent that the same configuration can be applied to the case of FIG. For details, refer to Japanese Patent Application No.
59355, Japanese Patent Application No. 2-259357, Japanese Patent Application No. 3-
No. 2378, Japanese Patent Application No. 3-2379, Japanese Patent Application No. 3-646
No. 23, Japanese Patent Application No. 3-64624, Japanese Patent Application No. 3-1583
See the specification of No. 30.

As described above, the mathematical principle of the depth analysis using the photoacoustic spectroscopy and the Fourier spectroscopy of the present invention is as follows.
Applicant's Japanese Patent Application Nos. 1-230209 and 1-33.
5748, Japanese Patent Application No. 2-259355, Japanese Patent Application No. 2-2
59357, Japanese Patent Application No. 3-2378, Japanese Patent Application No. 3-23
No. 79, Japanese Patent Application No. 3-64623, Japanese Patent Application No. 3-6462
No. 4, Japanese Patent Application No. 3-158330, etc., explained that it is the same as the mathematical principle of the time-resolved spectroscopy method, and explained various examples from the analogy. Please refer to the specification and drawings of these applications for other modifications and the like.

In the above description, it is assumed that the microphone 5 is used as the PAS unit 6 which absorbs the light irradiated from the sample 4 and converts it into heat, and detects the diffusion of the heat to the surrounding air. However, a change in the refractive index of air due to heat may be used to detect the thermal diffusion. That is, FIG.
As schematically shown in the AS unit 6 ', the laser light 20 is passed close to the surface of the sample 4 and the passed light is made incident on a one-dimensional or two-dimensional array detector 21 such as a CCD. deep. When the sample 4 is irradiated with pulsed light from the pulse light source 1 via the interferometer 2 and the sample 1 is heated from the inside by absorption, heat is diffused on the surface according to the time after the pulsed light irradiation and the air expands. Then, the refractive index changes according to the temperature. Then, the optical path of the laser beam 20 passing closely along the surface of the sample 4 is bent to a higher refractive index, and the light receiving position on the array detector 21 changes. This position change is converted into a signal by the array detector 21 and
It can be input to the preamplifier 7 in FIG. In this case, since the change in the refractive index can be detected very sensitively, the detection sensitivity is further improved.

Although several embodiments have been described above, the present invention is not limited to these embodiments, and various modifications are possible.

[0029]

As described above, according to the depth analysis method using the photoacoustic spectroscopy and the Fourier spectroscopy of the present invention, a pulse light source that emits light at a predetermined period is used as a light source, and the detection signal is a pulse light source light emission. Sampling at a later fixed delay time, extracting a predetermined range of frequency components of the sampled signal, performing a Fourier transform, and measuring a heat distribution at a specific depth corresponding to the delay time of the measurement sample depending on the irradiation light wave number. Since the information is obtained, the information in the depth direction of the sample can be accurately detected by utilizing the difference in the absorption characteristics in the depth direction of the measurement sample depending on the wave number of the irradiation light.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of an apparatus for carrying out one embodiment of a depth analysis method according to the present invention.

FIG. 2 is a view similar to FIG. 1 of another embodiment.

FIG. 3 is a diagram similar to FIG. 1 of an embodiment using a lock-in amplifier.

FIG. 4 is a view similar to FIG. 1 of an embodiment using a difference measurement method.

FIG. 5 is a view similar to FIG. 1 of another embodiment using a difference measurement method.

FIG. 6 is a diagram similar to FIG. 1 illustrating an embodiment of a difference measurement method without using a lock-in amplifier.

FIG. 7 shows an embodiment in which the apparatus of FIG. 1 is multi-channeled.
FIG.

8 is a diagram showing an embodiment in which the apparatus of FIG.
FIG.

FIG. 9 is a schematic diagram showing a configuration of another embodiment of the PAS unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Pulse light source 2 ... Interferometer 3 ... Timer 4 ... Measurement sample 5 ... Microphone 6 ... PAS unit 7 ... Preamplifier 8 ... Delay circuit 9 ... Gate circuit 10 ... Low-pass filter 11 ... Main amplifier 12 ... AD converter 13 ... Computer 14 ... Band pass filter 15 ... Lock-in amplifier 16 ... Multiplier

Claims (1)

(57) [Claims] 1. A method of irradiating a measurement sample with light modulated by a rapid scan interferometer, heat generated in the measurement sample is diffused and transmitted to surrounding gas, and a change in gas density based on the heat is detected. In a depth analysis method using photoacoustic spectroscopy and Fourier spectroscopy that convert the signal into an electrical signal and obtain the information in the depth direction of the measurement sample by Fourier transforming the signal, a pulse that emits light at a predetermined period as a light source using a light source, a detection signal is sampled at a constant time delay after the pulsed light source emitting retrieves frequency components in a predetermined range of the sampled signals, Fourier
D conversion, the delay of the measurement sample depending on the irradiation light wave number
A depth analysis method using photoacoustic spectroscopy and Fourier spectroscopy, which obtains heat distribution information at a specific depth corresponding to time .