1702
Anal. Chem. 1986, 58, 1702-1705
(12) K i m & F. M.; Baxter, J. P.; Winograd, N. Surf. Sci. 1083, 724, L41. (13) Walsh, A. Spectrochlm. Acta, Pad 6 1980, 358,639. (14) McNally, J. R.; Harrison, G. R.; Rowe, E. J . Opt. Soc. Am. 1947, 3 7 , 93. (15) Coburn, J. W. Rev. Sci. Instrum. 1970, 4 7 , 1219. (16) Chapman, 6. c. B O W Discharge Processes; Wiley-Interscience: New York, 1960. (17) Howatson, A. M. An. Introduction to Gas Discharges, 2nd ed.; Pergamon: New York, 1976. (18) Gerhard, W.; Oechsner, H. 2.fhys. 6 1875, 22, 41. (19) Harrison, W. W.; Bentz, B. L. Anal. Chem. 1979, 57, 1855. (20) Westwood. W. D. hog. Surf. Sci. 1976, 7 , 71. (21) Savlckas, P. J. Ph.D. Dissertation, University of Virginia, Charlottesville, VA, 1984. (22) Savickas, P. J.; Hess, K. R.; Marcus, R. K.; Harrison, W. W. Anal. Chem. 1984, 56, 817. (23) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 54, 1523. (24) Beekman, D. W. Ph.D. Dissertation. University of Tennessee, Knoxville, TN, 1983 p 17. (25) Fasset, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983, 55, 765. (26) Valyl, L. Atom and Ion Sources; Wiley: London, 1977; p 85. (27) Arnot, F. L. Collision Processes in Gases; Wiley: New York, 1957.
(28) Cobine, J. D. Gaseous Conductws; Dover: New York, 1958. (29) Carter, G.; Colllgon, J. S. Ion Bombardment of Solids; American Elsevier: New York, 1968. (30) Loeb, L. B. Fundamental Processes of Nectrical Discharge in Gases ; Wiley: New York, 1939. (31) Coburn, J. W.; Kay, E. Appl. fhys. Len. 1971, 18, 435. (32) Mehs, D. M.; Niemczyk, T. M. Appl. Spectrosc. 1981, 35, 66. (33) Brackett, J. M.; Mitchell, J. C.; Vlckers, T. J. Appl. Spectrosc. 1984, 38, 136. (34) De Galan, L.; Smith, R.; Winefordner, J. D. Spectrochim. Acta, Part6 1968, 236, 521. (35) Bruhn, C. G.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1979, 57, 673. (36) Moore, L. J.; Fasset, J. D.; Travis, J. C. Anal. Chem. 1984, 56, 2770. (37) Ion-Molecule Reactions; Lamper, F. W., Franklin, J. F., Eds.; Plenum: New York, 1972; Vol. 1. p 601.
RECEIVED for review November 8, 1985. Accepted February 18, 1986. Support of this research by the Department of Energy, Division of Chemical Sciences, is gratefully acknowledged.
Ultrasensitive, Isotopically Selective Detection of Nitric Oxide by Multiphoton Ionization in a Supersonic Beam John C. Miller Chemical Physics Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
~ a n k n b a # b n o f n i t f i c o w # e e x p a n d e d l ampmonlc n nozzle has been Investigated. Iootoprc selectivity is demo% strated by both wavdength sdectlon and tlmoof-f#ght mass spectrometry. SenatMtyarkestromthereeOnanttyenhanced three-photon knlzatlon of an ulbacoM nlklc oxide beam (- 1 K). By use of thls technique, 15Nib0, 14N180, and lsN1'O are detected In a natural abundance sample of nominally 14N150. I t is drown that on the average as few as 108-io4 mokicub of "N"O can be detected In a slngle laser shot. The extenslon of thls work to "slnglemolecule detection" Is discussed.
Since the observation by Veronov and Delone (1) that tightly focusing a ruby laser into xenon could produce ions (requiring at least 11photons), the field of multiphoton ionization (MPI) has expanded rapidly. The advent and use of powerful, tunable lasers have shown that resonant enhancement at some multiple of the photon energy can increase the MPI cross section by several orders of magnitude. Consequently, detection of ions while tuning such a laser effectively maps out the energies of allowed transitions. Thus, resonantly enhanced MPI has proven to be a versatile spectroscopic tool. Several advantages of MPI include different selection rules for multiple photon absorption, freedom from many experimental difficulties of vacuum ultraviolet spectroscopy, the translation of desirable laser properties (bandwidth, coherence, etc.) to the study of high-lying transitions, and the high efficiency of ion and electron detection. It is this last factor that spawned the subfield of resonance ionization spectroscopy (RIS) as defined and described in detail by Hurst et al. (2). Since ions and electrons can be detected with unit efficiency, if every atom in the laser beam can be ionized with unit efficiency then "single-atom detection" is possible. This can be achieved by saturating every bound-bound transition in the excitation scheme and pro-
viding sufficiently rapid ionization to saturate ionization out of the excited state. Single-atom detection was first observed by Hurst et al. (3, 4 ) for cesium. In contrast to atoms, molecules possess vibrational and rotational degrees of freedom that complicate and congest electronic spectra. New channels such as predissociation exist for the disposition of excitation energy. High photon fluxes may further complicate the physics and chemistry involved. The central problem, however, with ultrasensitive laser detection techniques as applied to molecules is the very large number of low-lying vibrational and rotational levels that may be populated in a room-temperature gas. A moderate resolution laser can then only interact with a small fraction of the available molecules. A demonstration of "single-molecule detection" thus requires cooling the molecules into a single quantum state, which is easiest for a diatomic molecule with a large vibrational frequency. The addition of mass spectrometry to molecular MPI studies has allowed isotopically selective detection. Several recent studies demonstrated the detection and spectroscopic investigation of the 13Canalogues of benzene (5), aniline (6), toluene (7), fluorobenzene (7),and several isotopic versions of nitric oxide in their natural abundance (8, 9). Studies of chlorine and bromine isotopes in substituted aromatic hydrocarbons have also appeared (10). The ultimate sensitivity of molecular MPI has been previously discussed (11, 12). In the present work supersonic expansion cooling and low-order MPI are employed in order to investigate the sensitivity of MPI detection of nitric oxide. Various isotopic versions of nitric oxide are used in their natural abundance to supply small but well-known concentrations. The extension of this work to a demonstration of "single-molecule detection" is discussed.
EXPERIMENTAL SECTION Various aspects of the experimental apparatus have been described in detail previously (13-15). Briefly, an excimer-pumped
0003-2700/66/0358-1702$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986 I
ROTATIONAL DISTRIBUTION FOR NITRIC OXIDE ( * T T + / 2 )
3375
I
I
I
3380
338 5
3390
339 5
340 0
1703
3405
LASER WAVELENGTHlnml
Figure 2. Mass-resolved multlphoton Ionization spectra for several lsotoplc variations of nitric oxlde present in their natural abundance. 5
0
45
40
20
ROTATIONAL QUANTUM NUMBER ( N )
f00
-
v)
y
00
3
u W 1
B 5
60
40
W 0
6
n
interact with 5% of the molecules. This value of 5% takes account of those molecules in the lowest excited electronic state of nitric oxide. The 2113/2state lies 120 cm-* above the ground state and at room temperature contains 35% of the total population. In order to increase the number of molecules in a given level, it is necessary to cool the gas. Figure 1 also shows Boltzmann distributions for temperatures of 10,5,and 1K. Even at 10 K one can only access -40% of the molecules a t a single wavelength. Cooling to about 1K is necessary to obtain 99.85% of the molecules in the lowest rotational level of the ground state. A further advantage of cooling is the large reduction in the number of possible transitions as the number of populated levels is reduced. This leads to an increase in specificity that would be useful in analyzing gas mixtures. As the vapor pressure of nitric oxide becomes small below about 100 K, direct cryogenic cooling of a sample is not practical for RIS detection of this molecule. Thus, supersonic expansion techniques are necessary to reach lower temperatures. These techniques have been outlined in numerous review articles (see, for instance, ref 16). In the present apparatus, temperatures of -10 K can be achieved with neat nitric oxide, and cooling to 1K is possible by expanding a 5% mixture of nitric oxide in argon (14). The two-photon resonant four-photon (2 2) MPI spectrum via the A 2 F (u = 0) state shown in ref 14 shows only three rotational peaks, the SZ1(1/2), S11(1/2) R21(1/2), and the QZ1(1/2)+ RI1(1/2) transitions, all originating from the lowest rotational level of the ground electronic state. Supersonic cooling also results in formation of clusters of the form Ar,(NO), which have been detected and studied separately (15). As the most abundant of these clusters, ArNO, is present at less than the 1% level there is no practical consequence to the experiments reported here. B. Isotopic Selectivity. For the present study, the (2 1)MPI resonant with the C2113/2,1/2(u = 3) state was chosen, in order to provide higher sensitivity by reducing the number of photons required to ionize nitric oxide. The (1+ 1)ionization via the A state would further increase the sensitivity, but the doubling crystal required to achieve the proper wavelength is easily damaged and has a low conversion efficiency. The third vibrational level was chosen to ensure a large isotope shift for maximum isotopic selectivity. The MPI spectrum obtained by time gating the time-of-flight ion signal for 14N160,15N160,14N180,and 15N1s0and tuning the laser is shown in Figure 2. The isotopes are present in their natural abundance in a 5% nitric oxide-argon mixture expanded through the supersonic nozzle. Previous studies of two-photon resonances with 211 states of nitric oxide show them to be dominated by Q branch lines (17,18).Such is the case for the u = 3 transition to the C211 state shown in Figure 2. Each isotopic spectrum is composed of essentially two peaks, which represent the Q21(3/2) and Qll(1/2) transitions. The transitions orginate from the J = 3/2 and 1/2 levels of the X2ll1/,state and terminate on the C21&/2 (J= 3/2) and C2111,2(J= 1/2) levels, respectively. The dominance of these lowest J transitions indicates a rotational
20
0 0 4 2 3 4 5
0 4 2
0 4
ROTATIONAL QUANTUM NUMBER ( N )
Flgure 1. Results of a Boltzmann calculation of rotatlonai state populations for the ground state of nltrlc oxide.
dye laser (Lambda Physik EMG 101, FL 2000E) is tightly focused (75 mm) 25-50 mm downstream of a pulsed supersonic nozzle source (Quanta-Ray PSV-2). Typical values of dye laser energy and power density are 10 mJ and 1O'O W/cm2, respectively. The pulsed nozzle delivers gas pulses of 60-100 1 s duration at backing pressures up to 10 atm. The source is typically operated as a free-jet expansion (i.e., no skimmer is used) at 10 Hz with a 0.5mm aperture. The laser is triggered externally after a variable delay with respect to the valve opening. Under typical operating conditions nitric oxide rare-gas clusters are also present in small concentrations. Spectroscopic studies of these clusters are published elsewhere (15). Following MPI, the ions are detected and characterized in a :shorttimeof-fight mass spectrometer having a resolution of about 50. That is, unit mass resolution is achieved in the mass region of interest for nitric oxide. Spectra are typically recorded by time gating the mass of the specific isotope of nitric oxide and tuning the laser. The signal from the high-current channeltron (Galileo Electro-Optics) is then amplified and averaged in a boxcar integrator (Stanford Research). Nitric oxide-argon mixtures of 1% and 5% composition were obtained from Matheson and used as received. RESULTS AND DISCUSSION A. Rotational Cooling. The foremost problem in molecular ultrasensitive detection is the inability of a narrowbandwidth laser to interact with all of the molecules at a single wavelength. The additional rotational and vibrational degrees of freedom of molecules provide a multitude of low-energy quantum states which, in room-temperature samples, are inaccessible to a laser tuned to a particular level. For instance, for nitric oxide at room temperature, a calculation based on a Boltzmann distribution of rotational levels is shown in Figure 1. There, the percentage of the total number of molecules that are in a given rotational quantum state is plotted as a function of the rotational quantum number, N , for the ground 2111/2state of nitric oxide. Note that the lowest rotational level, N = 0, corresponds to a total angular momentum quantum number, J = 1/2. It can be seen that the largest population is in the seventh rotational level (J = 7.5). However, a laser tuned to a transition originating from this level will only
+
+
+
1704
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Table I. Energies (cm-I) of the Observed Transitions of the Isotopic Nitric Oxide Molecules Qzi
A
this work
(1/2) previous observation=
1
59202 59016 58924 58730
59202 59021 58932 58734*
(3/2)
QII
previous
isotope
this work
observation"
14Ni60
59242 59054 58965 58773
59243 59059 58971' 58772'
15N160 14N'80 iaN'80
'Reference
23.
5
6 1
A
0 5 8
4
*Unpublished, ref 20.
temperature of 1-5 K. The energies of the peaks of Figure 2 are compared with previous observations in Table I. The spectrum of the 14N160isotope appears to be slightly warmer, however, as other rotational transitions can be observed. Alternately, the very large intensity of this transition for the normal isotope probably resulted in some saturation or space charge effects on the two largest peaks, thus making the higher J transitions appear relatively more intense. In principal the relative abundances of each isotopic species should be reflected in the relative intensities of each of the corresponding spectra. In practice, two factors render this comparison difficult. Because of the extremely small focal volume required for multiphoton absorption, the ion collection efficiency is sensitive to space charge effects. Hence, to accurately measure relative intensities or true spectral line shapes, it is necessary to make the measurements at comparable, rather low, ion densities. The various isotopic spectra of Figure 2 were recorded under optimum conditions of low ion density. The is, the laser was attenuated and the beam density reduced for recording spectra of the most abundant species, 14N160. Conversely, the weakness of the 15NlS0 spectrum required optimization of the laser power as well as fresh dye solution and a new gas fill in the excimer laser. In this case the laser power density was -1 X 1O'O W/cm2. As precise isotope ratios were not the purpose of the present work these fadon were not quantified. The second important factor is that all of the 211 states of nitric oxide are mutually perturbed (19). In particular, the C (u = 3) and the B (u = 15) states exhibit the strongest such homogeneous perturbations. As the details of the state mixing are isotope dependent, relative intensities of the various isotopic spectra do not offer a reliable measurement of isotope abundance (20). Furthermore, the isotope shifts are not simply calculated by the /~ ( p = reduced mass). Again, if accurate usual ( ~ / p J l factor isotopic determinations were required, a different, less perturbed, transition to the C211 (u = 0, 1,2) or the nearby D 2 P (u = 1, 0 ) states could be chosen. A final point concerns the 14N170isotopic species that has the same nominal mass as the 15N160molecule. The small mass difference (15N160= 30.99502 amu, 14N170= 31.00221 amu (21)) of these isobars certainly precludes direct mass discrimination except in high-resolution spectrometers. However, this small difference when "amplified" by the vibrational frequency of C state nitric oxide provides an easily observable isotope shift difference of about 30 cm-' in the transition energy for a Av = 3 transition. The 14N170spectrum should then appear to the blue of the 15N160spectrum (the reduced mass ( p = mlm2/(ml m2))of 14N170(7.67816 amu) is less than that of 15N160(7.74078 amu) in contrast to their total masses given earlier) by 30 cm-' but weaker by a factor of 10 (% abundance: I5N = 0.37, 170= 0.037) (21). Unfortunately, one of the expected peaks for 14N170would then lie under an 15N160peak and the other would occur in a region dominated by 14N160ionization detected a t mass 31 due to the poor mass resolution. Furthermore, the predicted positions
+
are uncertain due to the isotope-dependent perturbations mentioned above, and no previous measurements for this transition of 14N170have been published for comparison. A factor of 2 increase in both mass resolution and spectral resolution should be sufficient to separately detect the two isobars in the present apparatus. For the A22+ (u = 1)state of nitric oxide, the two spectra have been separately detected and assigned (9) using similar techniques involving roomtemperature nitric oxide. Again, if accurate isotope abundances were desired for these isobars, only minor improvements would be required to accomplish this goal. C. Sensitivity. The sensitivity of the isotope-selective MPI detection of nitric oxide can be estimated from the observation of the 15N180isotopic spectrum in a 5% nitric oxide-argon mixture with a signal-to-noise ( S I N ) ratio of about 2. This corresponds to a count rate of 1-2 mass-selected ions per laser shot when the laser is tuned to the 15N180resonance. As the collection efficiency of ions is unity, improvement in the S I N can only come from improved mass resolution, thus removing counts due to nonresonant ionization of the more abundant isotopes or resonant ionization of warm background gas. As mentioned previously, a factor of 2 improvement in mass resolution should eliminate this problem. The S I N is then determined by the gain of the channeltron detector. The natural abundances of 15N and l80are, respectively, 0.37% and 0.204% (21). Consequently in a 5% mixture of nitric oxide in argon, the fractional 15N180concentration is 3.8 X or about 380 ppb. It is more relevant, however, to estimate the number of 15N1S0molecules in the focal volume of the laser. The number density, p , at some distance, x , from a nozzle of diameter D is given by (22)
where p o is the total pressure behind the nozzle and 6' is the angle between the point of interest and the beam center line. For a 5 atm backing pressure, and an x / D of 40 ( x = 5 cm, D = 0.075 cm) the total number density at the laser focus is about 1.2 X 10l6species/cm3 or slightly less than 1torr. The number density of 15N"0 is correspondingly 4.6 X lo9 molecules/cm3. For a multiphoton process most of the ionization will originate only from the focal region whose volume can be approximated by
V=
TW:
b
where wo is the beam waist radius a t the focus and b is the confocal parameter. Assuming a diffraction-limited beam and the relevant experimental parameters, the focal volume is calculated to be 2.5 X lo-' cm3. Hence there would be about 800 molecules of 15N180in the focal volume. Since our laser is not diffraction-limited nor is the lens corrected for aberrations, the actual focal volume is probably a factor of 10 larger. In the present experiment, then, 103-104 molecules are available in the sample volume, and on the average 1-2 of these are ionized by the laser and detected on each laser shot. The ionization efficiency is thus on the order of to ions/molecule. The detection limit for this technique with the present apparatus is within a few orders of magnitude of the ultimate sensitivity-single-molecule detection. D. Extension to "Single-MoleculeDetection". A demonstration of single-molecule detection would require the following elements. First, the laser must be able to interact with all of the molecules at a single wavelength. As shown in the present work, this can be accomplished by supersonic cooling to -1-2 K. Next, the ionization scheme must have unit probability of producing an ion-electron pair, and the detection scheme must be able to detect a single charged
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
particle. Single isotope selectivity by a combination of mass spectrometry and choice of laser wavelengths has been demonstrated in the present work. Commercially available particle multipliers such as the one used here are capable of fulfilling the latter requirement. The former requirement has been discussed in detail for laser ionization of atoms in the review of Hurst et al. (2). All bound-bound transitions must be saturated, and then the ionizing process must be fast relative to other decay paths. In the present experments, the measured power dependence is slightly less than 3, indicating partial saturation, probably of the ionizing step. The two-photon transition to the C state is not saturated at all. Either higher power or a tighter focus should lead to saturation. This condition would increase the ionization efficiency from the value of or so demonstrated here to unity. Alternatively, one could use nonlinear optics techniques to reach the intermediate state in a one-photon process, which could easily be saturated. Molecular transitions are somewhat harder to saturate than atomic transitions because of the reduced oscillator strength density due to the vibrational and rotational degrees of freedom. Furthermore, most states of nitric oxide, including the C2nstate, exhibit predissociation to some degree. Once saturation is achieved, the gas density can be simply reduced until, on the average, only one molecule is present in the laser focus. The results of the present experiment indicate that there are no fundamental obstacles to detection of a single isotopically selected nitric oxide molecule. Either simple scaling to higher power or a reduction of the (2 1) ionization to a (1 + 1) scheme is required.
+
CONCLUSIONS The emphasis in this work has been an evaluation of the sensitivity of MPI detection of nitric oxide. The ultimate goal was to demonstrate all of the elements for a "single-molecule detection" scheme. The present studies show that there are no fundamental obstacles to this goal. Because of this emphasis, supersonic cooling was essential in order to concentrate all of the molecules in a single quantum state. However, similar isotope-selective experients can be performed much
1705
more simply in a static cell configuration with similar sensitivity if single-molecule detection is not sought. In this case, the many complications of a molecular beam apparatus and supersonic cooling should be avoided. Registry No. 15N160,15917-77-8;14Ni80,15917-78-9;15N180, 15917-79-0;NO, 10102-43-9.
LITERATURE CITED (1) Veronov, G. S.; Delone, N. B. JETP Lett. 1065, I , 66. (2) Hurst, G. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1070, 51, 767. (3) Hurst, G. S.; Nayfeh, M. H.; Young, J. P. Appl. Phys. Lett. 1077, 3 0 , 229. (4) Hurst, G. S.; Nayfeh, M. H.; Young, J. P. Phys. Rev. A 1077, 15, 2283. (5) Boesl, U.; Neusser. H. J.; Schlag, E. W. In Laser Spectroscopy I V ; Walther, H.; Rothe, K. W., Eds.; Springer: Berlln, 1979; Vol. 21, p 164. (6) Leutwyler, S.; Even, U. Chem. Phys. Lett. 1081, 81, 578. (7) Dimopoulou-Rademann, 0.; Rademann, K.; Brutschy, B.; Baumgartel. H. Chem. Phys. Lett. 1083, 101. 485. (8) Zacharias, H.; Schmiedl, R.; Welga, K. H. Appl. Phys. 1080, 21, 127. (9) Zacharlas, H.; Rottke, H.; Welga, K. H. Appl. Phys. 1081, 2 4 , 23. (10) Lubman, D. M.;Tembreul, R.; Sln, C. H. Anal. Chem. 1085, 57, 1064. (11) Klimcak. C.; Wessel, J. Appl. Phys. Lett. 1080, 3 7 , 138. (12) Antonov, V. S.; Letokhov, V. S. Appl. Phys. 1081, 24,89. (13) Cooper, C. D.; Wliamson, A. D.; Miller, J. C.; Compton, R. N. J . Chem. Phys. 1080, 7 3 , 1527. (14) Mllier, J. C.; Compton, R. N. J. Chem. Phys. 1086, 8 4 , 675. (15) Miller, J. C.; Cheng, W. C. J. Phys. Chem. 1085, 89, 1647. (16) Levy, D. H. Annu. Rev. Phys. Chem. 1080, 3 1 , 197. (17) Freedman, P. A. Can. J. Phys. 1077, 55, 1387. (18) Achiba, Y., Sato, K.; Kimura, K. J. Chem. Phys. 1985, 82, 3959. (19) Galusser. R.; Dressier, K. J . Chem. Phys. 1082, 76. 4311. (20) Miescher, E., private communlcatlon, 1965. (21) Handbook of Chemistty and Physics, Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1967. (22) Levy, D. H.; Wharton, L.; Smaliey, R. E. I n Chemical and Biochemical Applications of Lasers; Moore, C. B., Ed.; Academic Press: New York, 1977; Voi. I, p 1. (23) Lagerqvist, A.; Miescher, E. Helv. Phys. Acta 1058, 3 1 , 22.
RECEIVED for review November 22, 1985. Accepted March 5 , 1986. Research sponsored by the Office of Health and Environmental Research, U.S.Department of Energy, under Contract DE-AC05-840R-21400with Martin Marietta Energy Systems, Inc.
