New method for measuring low NO concentrations using laser induced two
photon ionization
Shan-Hu Lee, Jun Hirokawa, Yoshizumi Kajii, and Hajime Akimotoa)
Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro,
Tokyo 153, Japan
~Received 28 January 1997; accepted for publication 27 April 1997!
A new method based on laser induced two photon ionization was developed for detection of low
levels of NO concentration in the troposphere. A frequency-doubled pulsed dye laser operating at
near 226 nm was used to photoionize NO by a (111) resonance enhanced two photon ionization
via its A 2 S←X 2 P(0,0) band. NO ions are detected by an electron multiplier. Air gas samples are
injected through a pulsed nozzle into a vacuum chamber. Cooling of rovibrational levels during the
expansion process allows better selectivity for NO detection from other nitrocompounds.
Measurement conditions were optimized as a function of X/D for the nozzle ~X is the distance from
the nozzle orifice to the ionization area, and D is the diameter of the nozzle orifice!, laser power, and
electron multiplier voltage. For the first time, selectivities for NO measurement, which were
obtained from ion intensity ratios of NO and other nitrocompounds such as NO2 at the same
concentrations, are discussed. A selectivity of 45 for NO vs NO2 was obtained using a 44 mJ pulse
energy and an X/D of 120 for the nozzle. The current detection limit determined for NO was 16
pptv with an integration time of 1 min and a pulse energy of 44 mJ. © 1997 American Institute of
Physics. @S0034-6748~97!04807-7#
I. INTRODUCTION
NO plays an important role in atmospheric chemistry,
mainly in the chemistry of ozone. In the troposphere, in particular, the photochemical production of ozone critically depends on the concentration of NO.1 NO is also one of the key
species in the formation of HNO3 which is a major component in the acidic precipitation. It is therefore necessary to
measure low NO concentrations in order to confirm photochemical theories in the troposphere and to correctly estimate the increase of tropospheric oxidants directly contributed by anthropogenic influences.
Intensive research has been conducted to develop the
technology for measuring low NO concentrations.2,3 Among
these, the most common method of NO detection is based on
the chemiluminescent reaction with ozone (CL–O3);
4–7
O31NO→NO*
This reaction produces electroni2 1O2.
cally excited NO*
2 with an emission intensity proportional to
the initial concentration of NO. Another method to detect
NO is two photon laser induced fluorescence ~TP-LIF!.8,9 In
this method, a 226 nm and a 1.1 mm laser beam were used to
sequentially excite the rotationally resolved transition in the
D 2 S←A 2 S and A 2 S←X 2 P manifolds, and the fluorescence resulting from the D 2 S→X 2 P transition was detected. Since the fluorescence occurs near 190 nm, a wavelength region free from laser generated background noise,
signal-limited ~SL! measurement conditions in NO determination were obtained. Both measurement methods have
achieved a detection limit in sub-pptv levels. Other methods
include long path absorption and differential optical absorption which determined the relatively higher NO concentrations in urban pollution air.10–12
a!
Author to whom correspondence should be addressed; Electronic mail:
akimoto@atmchem.rcast.u-tokyo.ac.jp
Rev. Sci. Instrum. 68 (7), July 1997
In this work, a new method for measuring low NO concentrations was studied by using laser induced two photon
ionization, which is different from above methods, with the
purpose of measuring tropospheric NO. A frequencydoubled pulsed dye laser operating near 226 nm was used to
photoionize NO by (111) resonance enhanced two phonon
multiphonon ionization ~TP-MPI! via its A 2 S←X 2 P(0,0)
band. The general concept of ionization spectroscopy is well
established.13–16 The ionization method possesses significant
advantages over the LIF method in detection efficiency.17–19
In the ionization method, close to 100% of the ions can be
collected, while in LIF the detection efficiencies are about
1024 due to the isotropical emission of photons. In addition,
collisional quenching and reactions of electronically excited
NO with other gases decrease the detection efficiency in LIF.
Laser photoionization spectroscopy has been proposed
as a method for detecting atmospheric pollutants like aniline,
chlorinated ethylene, nitrobenzene, and dimethyl sulfide in
ppmv or ppbv levels and is summarized well by Simeonsson
et al.17 However, little data pertaining to the quantitative determination of low concentration of NO in the atmosphere
are available. Simeonsson et al.17 and Lemier et al.18 have
attempted to use this ionization system to develop a detection
method for the measurement of NO and also of a number of
NO2-containing nitrocompounds ~i.e., NO2, HNO3, nitromethane, DMNA, nitrobenzene, TNT, and RDX! using
photofragmentation/photoionization processes. An ArF laser
operating at 193 nm was used to fragment the target nitrocompounds and the resultant NO was again photoionized via
its D 2 S←X 2 P(0,0) band; for NO detection, NO was directly photoionized. They have also used a 226 nm dye laser
for both the fragmentation and ionization via the
A 2 S←X 2 P(0,0) band of NO. Using the molecular beam
~MB! technique and time of flight mass spectroscopy ~TOFMS! a better detection limit of 8 ppbv was obtained using the
0034-6748/97/68(7)/2891/7/$10.00
© 1997 American Institute of Physics
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226 nm laser with an integration time of 10 s compared to
1.2 ppmv using the 193 nm laser, in spite of the lower laser
energy ~100 mJ at 226 nm and 1–1.5 mJ at 193 nm!. This
result is apparently due to the fact that the 193 nm excitation
is resonant with the transition from weakly populated rotational states in the vibrational ground state in the A←X band
of NO. They have further improved the detection limit to 1
ppbv for NO determination at 226 nm with a simplified detection apparatus by using a pair of miniature electrodes for
ion detection, since the absolute molecular number detected
was enhanced over the molecular beam technique. They suggested that exclusive NO ion production at 226 nm made
mass detection by the time of flight mass spectrometer unnecessary for NO ion detection. By using (211) photon
ionization of NO via its C 2 P( v 50) state, Guizard et al.
have measured NO concentrations in laboratory study.19 The
purpose of the present study is to focus on the selective detection of NO with a high sensitivity by minimizing the spectral interference from other nitrogen oxides, particularly
NO2, as opposed to focusing on the nonselective detection of
all nitrocompounds by using the same photodissociation/
photoionization process reported by Someosson et al.
In order to improve the selectivity of NO, a pulsed
nozzle was used to expand gas samples in the vacuum chamber. Supersonic expansion provides rotational cooling of the
sample which significantly simplifies its spectrum by reducing the congestion from hot band absorption.20,21 Higher selectivity for NO detection is thus allowed by using the nozzle
to expand the sample gases. Other nitrogen oxides ~i.e.,
NO2, HNO3, N2O5! initially contained in the air gas samples
are also photofragmented into NO.22,23 However, such photodissociated NO has higher rovibrational levels. By using an
appropriate laser wavelength, the lower rovibrational levels
of the originally contained NO can be selectively detected.
At the same time, if a molecule is rotationally cooled, a
higher detection efficiency can be obtained since the cooled
molecules are concentrated in the lower rotational levels and
can be excited more effectively using a single wavelength of
light.19,20 In order to compensate for the decrease in the absolute molecular number density in the nozzle beam, ion
collection was performed by using an electron multiplier in a
vacuum ionization cell.
II. EXPERIMENT
Figure 1 presents a schematic drawing of the experimental apparatus used to detect low NO concentrations. A second harmonic of a pulsed Q-switched Nd:YAG laser
~Lambda Physik, LP 150, 5 ns duration! optically pumped a
dye laser ~Lambda Physik, Scanmate 2EC!. The dye laser
was frequency doubled to provide tunable ultraviolet ~UV!
radiation in the region of 225–227 nm ~Coumarin 47!. With
a repetition rate of 20 Hz and a pulse width of 0.13 cm21, a
maximum energy of 500 mJ was obtained. UV laser power
was monitored using a joulemeter ~Gentec, ED-100! as well
as a photodiode. The output beam was directed to the ionization sampling cell by prisms and was passed to the central
ionization region located below the nozzle and in front of the
electron multiplier ~Murata, Ceratron-E, EMS-6081 B!. The
laser beam is perpendicular to the molecular beam and to the
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Rev. Sci. Instrum., Vol. 68, No. 7, July 1997
FIG. 1. Schematic of the experimental apparatus.
electron multiplier head. Operation of the pulsed nozzle and
the laser was controlled by a pulse generator ~General Valve,
IOTA ONE!.
The ionization cell ~20 l volume! was pumped with a
turbomolecular pump ~Varian, TP 550 l/s! to keep a background pressure of 131028 Torr. Even when the pulsed
nozzle ~General Valve, 0.5 mm diameter, 20 Hz, 250 ms
duration time! was operated to inject sampling gases into the
ionization cell, a background gas pressure of 1025 Torr
could still be maintained. Due to these vacuum conditions,
the electron multiplier was able to be used for charge collections with high ion-collection efficiencies ~;107 gain at 2.5
kV!. The electron multiplier was located 2 cm from the laser
beam so that all ions generated by the radiation could be
collected. To estimate the cooling effect, ionization spectra
were taken under various X/D conditions (X/D:40– 200) by
changing the distance from the nozzle orifice to the ionization area (X) with a vertical manipulator ~Kitano Seiki,
KMUD-34! where D corresponds to the nozzle diameter.
The ionization signals measured by the electron multiplier were amplified with a gain of 125, discriminated from
the background noise, and further counted by the counting
units ~Hamamatsu, C3866 and M3949!. A delay of 1 ms
between the laser pulse and the appearance of the ion signal
allows effective separation of the ion signal from the scattered laser light pulse. The ion counts which correspond to
the original ion signals were integrated for 5 s at 20 Hz and
averaged over 100 laser shots. A personal computer was used
for storage and data analysis. The background ionization @;5
CPS ~counts per second! at laser power of 44 mJ# was estimated by scanning the laser wavelength in the off-resonance
region. The limiting noise is mainly caused by fluctuations in
the nonresonant background ionization.
Sample gases with different NO and NO2 concentrations
were obtained by diluting 10 ppmv NO or 5 ppmv NO2 in
pure N2 gases ~all Nihon Sanso, .99.9999% purity for
N2!. Figure 2 illustrates the dilution system for NO detection.
Four mass flow controllers ~MKS, absolute accuracy of 1%!
were used for dilution of the standard gases in two successive steps. At the final step, a NO concentration of 50 pptv
was obtained. To eliminate contamination caused by surface
adsorption,24 the internal surfaces of the calibration lines and
Low NO concentrations
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FIG. 2. Gas supply system for NO MPI measurement. MFC: Mass flow
controller; RP: rotary pump; PG: pressure gauge.
supply lines were coated with Teflon and were baked up to
130 °C before being calibrated.
III. RESULTS AND DISCUSSION
A. Optimization of the experimental conditions
Figure 3 shows a typical NO ionization spectrum obtained when 100 ppbv NO/N2 gas was injected into the ionization cell using 44 mJ laser pulse energy and a X/D of 45.
The A 2 S1←X 2 P 1/2 and A 2 S1←X 2 P 3/2 subbands are
apparently separated. Each rotational line was assigned as
the transition of NO from the X to the A state by using NO
molecular constants25–27 and referring to the known NO ionization spectra.28–30 Figure 4 is the expanded portion of the
NO ionization spectrum in the region of 226.230–226.410
nm showing the assignment for the rotational transition in
the A 2 S1←X 2 P 1/2 subband. In this work, the laser wavelength was chosen to excite the P 21(1/2) line with the
A 2 S1←X 2 P 1/2 transition of NO.
In order to optimize the experimental conditions, NO
ionization was measured by using the signal to noise ratio
~SNR! as a function of laser power, X/D for the nozzle, and
electron multiplier voltage. During the measurements, the
laser power varies significantly due to the shot-to-shot fluctuations and the difference in dye efficiency during the scanning. Accurate NO concentrations, therefore, could be obtained from the MPI intensities that had been normalized to
the laser power. In general, for the two photon ionization
FIG. 3. 111 resonance enhanced multiphoton ionization ~REMPI! spectrum of the NO ~0,0! band recorded at 100 ppbv NO/N2 gases.
Rev. Sci. Instrum., Vol. 68, No. 7, July 1997
FIG. 4. Small part of the of 111 resonance enhanced spectrum of the NO
A S←X 2 P 1/2 ~0,0! band showing results of the assignment for the rotational transition lines.
process power dependence follows a purely quadratic behavior ~two photon transition!31 or linear behavior ~saturation of
one step!.32–34 The two photon resonant enhanced multiphoton ionization of NO can be described by
dn 0 /dt5 s I @ n A ~ t ! 2n 0 ~ t !# 1n A ~ t ! g ,
~1!
dn A ~ t ! /dt5 s I @ n 0 ~ t ! 2n A ~ t !# 2 s i In A ~ t ! 2n A ~ t ! g , ~2!
dn i ~ t ! /dt5 s i In A ~ t ! ,
~3!
where n 0 (t), n A (t), and n i (t) are the population densities in
the ground state, the intermediate A state, and the ionic state,
respectively ~n 0 5N 0 and n c 5n i 50 at t50!; I is the photon
flux;
s (6310219 cm22)
and
s i @ (7.060.9)310219
22 35
cm ] are the cross sections for the one photon excitation
and the one photon ionization process, respectively; and g is
the effective rate for the other population loss process for the
resonant A state. The time dependent equations are considerably simplified by making the assumption that the laser
pulse is approximated by a step function with height I between 0 and t, i.e., during the pulse duration. If the steadystate population in the intermediate level A is assumed, i.e.,
dn A (t)/d(t)50, the number of ions produced after each laser shot is given by
n i ' s I 2 N 0 t s i / ~ s i I1 g ! .
~4!
Therefore, if the laser intensity I is strong enough, i.e., s i I
@ g , then the ionization power dependence will be linear; on
the other hand, if I is weak enough, i.e., s i I! g , then the
ionization power dependence will be quadratic.
Figure 5 shows the results of laser power dependence
obtained from the Q 111 P 21(1/2) line of the NO
A 2S←X 2 P 1/2(0,0) band on a logarithmic scale. From a
least-square fit, it can be seen that the MPI intensity depends
on the laser intensity as S} P 1.75, where S and P correspond
to MPI intensity and laser power, respectively. This result
indicates that the two photon NO ionization process is a
‘‘partial saturation’’ transition for laser powers of 27–55 mJ.
The laser power of 40 mJ used corresponds to a photon flux
of 6031024 cm22 s21 with an average laser spot size of
4.5 cm2. This result agrees well with that reported by Jacobs
et al.36 They studied 111 resonance enhanced MPI spectra
Low NO concentrations
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FIG. 7. Variation of the NO ion signal intensity, noise, and signal-to-noise
ratio ~SNR! with the high voltage setting on the electron multiplier.
FIG. 5. Laser power dependence of the NO ion signal for the Q 11
1 P 21(1/2) transition of the NO A 2 S←X 2 P 1/2 ~0,0! band.
by using the same NO A 2 S←X 2 P(0,0) system, and also
observed a noninteger power dependence for NO ionization,
1,n,2. According to this particular power dependence of
NO ionization, it is essential to monitor laser power during
NO-field observations.
Figure 6 shows the relationship of the NO ion intensity
to the X/D for the nozzle. For both the P 211Q 11(1/2) and
the P 221Q 11(1/2) branches, NO ion intensities increase with
a X/D increase in the region of 35–45, and they then decrease with decreasing X/D. This observation can be explained by the fact that the number of NO molecules in the
ionization area varies with the distance of X for the nozzle.
In the beginning, the overlapping of the molecular beam and
the laser spot increased with the increase of X and, as a
result, the number of NO ions increased. After approaching
the maximum at a X/D of 45, the molecular density at the
laser spot decreases with an increase of X.
Figure 7 shows the variation of NO ionization intensity
as a function of electron multiplier voltage. At a given volt-
age ranging from 2.1 to 3.5 kV, NO ion intensity was enhanced significantly since both the ion collection efficiency
and detection gain became higher. On the other hand, the
discrimination level setting for signal counting suppressed
the noise level so that the noise strength was not increased.
As a result, the SNR increases almost linearly with increasing voltage. This indicates that no saturation of charge collections occurred in this voltage region up to 3.5 kV, so it
was possible to increase the election multiplier voltage to
enhance the sensitivity for NO detection.
B. Detection limit for NO determination
Figure 8 shows the linearity of the instrument for NO
calibration ranging from 100 ppbv to 10 ppmv. This calibration was carried out at high enough concentrations so that the
NO ionization intensity is much higher than the zero signal.
Variances in the signal intensity caused by laser power variations were normalized by monitoring laser power at the same
time.
The detection limit can be given approximately by
@NO#limit5~S/N!~ 2S bg! 1/2C 21 t 21/2,
~5!
where S bg , C, and t correspond to the background signal
intensity, the instrumental sensitivity, and the integration
time, respectively. In this study, the sensitivity averaged
about 10 CPS/ppbv, while the background signal was about
5 CPS. A detection limit of 82 pptv (S/N52) for NO deter-
FIG. 6. Relationship of NO ion intensity to X/D for the nozzle. The open
circles indicate the P 211Q 11 ~0.5! line and the closed circles the P 22
1Q 11 (1/2) line.
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Rev. Sci. Instrum., Vol. 68, No. 7, July 1997
FIG. 8. MPI calibration line for NO measurement.
Low NO concentrations
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mination was obtained with an integration time of 1 min
using laser power of 44 mJ, electron multiplier voltage of 2.2
kV, and X/D of 45 for the nozzle. As shown in Fig. 7, the
SNR for NO detection at an electron multiplier voltage of 3.2
kV becomes more than five times higher than at 2.2 kV.
Therefore, a better detection limit of 16 pptv could reasonably be obtained by setting a voltage of 3.2 kV on the ion
detector. While the atmospheric air was injecting at 20 Hz
with a nozzle opening time of 250 ms, the pressure of the
vacuum chamber was 131025 Torr. Under such conditions,
we have estimated a molecular density of about 1
31023 Torr in the nozzle beam and an absolute molecule
number of 131014 of the sample gases in the ionization
volume (5 cm3).
Compared to the detection limit of 1 ppbv (S/N53)
reported by Someonsson et al. in 1994 when using the same
two photon ionization system, the detection limit has been
improved by two or three orders of magnitude. This improvement may have been made possible by several factors,
namely, the absolute molecular density, the ionization volume, the photon flux, and the ion collection efficiency of the
detector. Although molecular density in this work is much
lower than that reported by Someonsson et al. ~100 Torr!,
our ionization volume is almost five orders larger since the
laser beam is not focused. Jacobs et al.,36 who have also
studied two photon resonance enhanced multiphoton ionization of NO using a 226 nm laser extensively, suggested that
tight focusing of the laser beam could cause severe problems, such as a large gradient of laser intensity in the ionization region. The improved detection limit obtained in this
work was achieved mainly by the high ion collection efficiency of the electron multiplier which produces much
higher ion collection gain compared to the simple parallel
electrodes that Someonsson et al. used. The low detection
limit of 16 pptv (S/N52) for NO determination obtained in
this study demonstrates that this laser induced two photon
ionization method is a powerful technique for measurement
of the tropospheric level of NO and ultimately for a better
understanding of photochemical ozone formation.
With the current experimental setup, however, the detection limit still critically depends on the intensity of background ionization signals, despite the fact that the nonresonant ion signals were minimized significantly by using a high
vacuum ionization cell. In order to fully discriminate the NO
ions from the background nonresonance ionization signals
and to further substantially enhance sensitivity, a new
TOF-MS is now being built. It would also be worthwhile to
use a larger nozzle diameter in order to increase the molecule
density in the ionization region.
C. Selectivity for NO detection
During the NO laser ionization, other nitrocompounds
~i.e., NO2, N2O5, HNO3! that exist in the atmosphere can
also produce NO ions through photofragmentation and
photoionization. First, all of these NO2-containing compounds are photolyzed to NO2 ~Ref. 18! via the process of
hn
R1NO2
RNO2 ——→
226 nm;
Rev. Sci. Instrum., Vol. 68, No. 7, July 1997
~6!
then NO ions are again produced as described below:22
hn
NO2→ NO2* ——→ NO~ X 2 P i , v 9 ,J 9 !
2h n
1O~ 1 D, or
1
3
P !,
2
NO~ X P i , v 9 ,J 9 ! →NO 1e .
2
~7!
~8!
Actually, these nitrocompounds were detected using an UV
laser at 226 nm to photofragment and photoionize these nitrocompounds via the A 2 S←X 2 P ~0,0! band of NO.17,18
The results show that the relative sensitivities of other nitrocompounds at 193 nm are in general higher than those at 226
nm since a large fraction of the nascent NO fragment population distribution is more easily resonant with the 193 nm
radiation than it is with the 226 nm radiation. Butler et al.37
and Moss et al.38 studied the population distribution of the
photofragmentations of nitromethane. Their results indicate
that up to 10% of the NO produced by dissociation of the
NO2 photofragment may be in v 9 51 in the X 2 P state.
Moreover, Morrison et al.22 reported that the NO vibrational
state of v 9 51 was accessible in the two photon photodissociation of NO2→NO( v 9 )1O( 1 D) at an excitation wavelength of 450 nm.
In order to eliminate interference from the photofragmentation of other nitrocompounds and to enhance selectivity for NO detection, a pulsed nozzle was used in this study.
In contrast to the jet cooled NO molecules, photofragmented
NO is produced in a wide distribution of rovibrational levels,
and hence, selective detection for NO can be obtained. Here,
only NO2 was used to study selectivity typical of nitrocompounds contained in the air.
A direct comparison of ionization spectra for NO and
NO2 was conducted in this study. Two spectra were recorded
under exactly the same experimental conditions by introducing gases of 100 ppbv NO/N2 and 5 ppmv NO2 /N2. As a
result, it was found that NO has a higher intensity of 55
compared with NO2 detection. This selectivity may be explained in terms of two factors: ~1! NO and NO2 have different ionization efficiencies due to the more complicated
photoionization process in NO2 compared to NO, and ~2! the
expansion process with the nozzle increases the population
in the ground level of rovibrational energy for NO molecules
which originally exist in the air, in contrast, NO molecules
photodissociated from NO2 are excited rotationally and vibrationally.
Since the ionization process for NO2 is three photon process compared to two photon process for NO, it is speculated
that ionization of NO2 should have a higher laser power dependence than that of NO. We have also investigated the
laser power dependence of the NO ions produced by NO2
photodissociation. In contrast to the result observed in NO
ionization, S} P 1.75 as shown in Fig. 5; NO2 ionization
showed a stronger power dependence, S} P 2.42, where S and
P correspond to MPI intensity and laser power, respectively.
Apparently, the ionization of NO2 involves a multiphoton
process that makes the ionization of NO2 more strongly dependent on laser power than NO. This indicates that the selectivity depends on laser power. It is also necessary to understand how the selectivity can be improved by using a
nozzle rather than by using the flow method to introduce gas
Low NO concentrations
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FIG. 10. Variation of rotational temperature with X/D for the nozzle. The
vertical lines show the uncertainties in the calculation of the rotational temperature from the Boltzmann plot.
FIG. 9. Comparison of the selectivity of NO vs NO2 for the nozzle and flow
at different laser powers. The selectivity corresponds to the ratio of the
ionization intensity of NO and NO2 with the same concentrations.
samples. Figure 9 shows the ion intensity ratios of NO and
NO2 as a function of laser power, introducing sample gases
with the nozzle and flow, respectively. The same concentration of NO and NO2 gases was used, and spectra of NO and
NO2 were taken under the same experimental conditions.
First, it was found that the selectivity increases with decreasing laser power. This result agrees quite well with the results
of differing laser power dependence for NO and NO2. Therefore, moderate laser power has to be chosen for a high sensitivity of NO detection and for a useful selectivity over
other nitrocompounds. Second, the experimental results
show the remarkably increased selectivity using jet expansion compared to use of the flow method. For example, a
selectivity of about 50 for NO vs NO2 was obtained by using
44 mJ of pulse energy and a X/D of 120 for the nozzle
compared to that of about 3 by using the flow method at the
same laser power. Since the selectivity is a function of laser
power, this high selectivity of 50 will be conserved even if
NO concentrations decrease as low as 16 pptv ~the detection
limit for NO determination! at a laser power of 44 mJ. This
demonstrates that for a selective measurement of NO a
nozzle is an effective tool for introducing gases.
In order to better understand the cooling effect of the
nozzle, the rotational temperature of NO was calculated from
the Boltzmann plot using the NO ionization spectra recorded
at different values of X/D and using the known energy level
formulas of NO and Hönl–London factors.26 As can be seen
in Fig. 10, the calculated rotational temperatures of NO are
between 35 and 25 K for X/D between 40 and 70. The temperatures obtained indicate an increase in population of relatively low rovibrational states of the electronic NO ground
level. This therefore demonstrates that the high selectivity
for NO resulted from the expansion process of the gas jet.
In summary, by using a nozzle, interference from the
other nitrocompounds can be reduced. Besides the cooling
effect, multiphoton dissociation and the ionization process
for these NO2-containing compounds are also responsible for
the significant selectivity for NO measurement. Although
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Rev. Sci. Instrum., Vol. 68, No. 7, July 1997
only NO2 gases were chosen for the selectivity experiment in
the present study, it may be expected that interference from
other nitrocompounds in ambient air may be smaller than
that from NO2 since they are all photodissociated to NO2
first. For example, HNO3 is photodissociated to NO2 primarily as39
hn
HNO3→ OH1NO2,
and therefore the ionization efficiency can be expected to be
much lower than that for NO2.
IV. DISCUSSION
A new method for the detection of low levels of nitric
oxide was developed for measurements of background NO
concentrations in the troposphere. The measurement apparatus used a 226 nm pulsed dye laser to ionize NO by resonance enhanced two photon ionization via its
A 2 S←X 2 P(0,0) band on a pulsed gas jet. The experimental conditions were optimized for high sensitivity NO detection as a function of laser power, electron multiplier voltage,
and X/D. The detection limit of NO was as low as 16 pptv
(S/N52). Using the pulsed nozzle to introduce sample
gases, improved selectivity over the flow method was obtained. The selectivity depends on laser power.
In order to eliminate the background ionization signal to
improve the NO measurement sensitivity, a time of flight
mass spectrometer is now being built. The improved spectrometer planned for use in the detection of background NO
at remote sites in Japan to obtain data for photochemical
modeling of the troposphere.
ACKNOWLEDGMENTS
One of the authors ~S.L.! would like to thank Professor
T. Kondow of The University of Tokyo for his continuous
encouragement during this work, and she is also grateful to
Dr. K. Hukutani and Dr. T. Tsukuta for their scientific and
technical advice. This work was partially supported by the
Japan Society for the Promotion of Science ~JSPS!.
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