Cavity ringdown spectra of theà 2 A −X 2 A electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both theà andX states of CH3O2·. An estimate for the absorption cross-section is determined from the CRDS absorption and the rate of radical-radical recombination.Low temperature oxidation of hydrocarbons is a pervasive process in both nature and technology. It is critically important for the environmental quality of our atmosphere. [1][2][3][4][5] Similarly it is critical to the efficiency and fuel economy of internal combustion engines. 6-8 Arguably the most important reaction 7 for low temperature oxidation is the production of peroxy radicals (RO 2 ·) from alkyl radicals (R·), i.e.,There are a number of important alkyl peroxy radicals defined by the nature of the R group, ranging 1 from the simplest species, methyl (R=CH 3 ) peroxy to complex species with R containing at least 8-10 carbon atoms with correspondingly many structural isomers. Because of their significance, much effort 1 has been expended studying the mechanisms and kinetics of peroxy radical production and their subsequent reactions. This work has largely been based upon monitoring of the peroxy radicals via theirB 2 A −X 2 A UV absorption. This is a strong transition, common to all the peroxies, that is centered around 240nm.Unfortunately this transition is broad and unstructured with a half-width of ≈40nm. The quasi-continuum nature of this transition has at least two clear disadvantages. It is unsuitable for obtaining rotational or vibrational information about the radical. Additionally the overlapping of the UV spectra of different RO 2 · radicals makes the identification of a specific alkyl peroxy radical, particularly from a mixture, a significant challenge.Experiments involving the peroxyà 2 A −X 2 A transition in the IR are sparse. There was an early report, 9 using low resolution modulated absorption spectroscopy, of the observation of theÃ−X IR transition of several RO 2 · radicals and more recently a report 10 of the observation of fragmentary spectra of ethyl peroxy using a cw absorption technique. There was also a report 11 of the detection of theÃ−X transition of CH 3 O 2 · by intracavity laser absorption spectroscopy, but few spectroscopic details were given. Based upon the recent observation 12 and analysis 13 of a well resolved spectrum of the hydroperoxy radical, HO 2 ·, we expect thẽ A −X IR transitions of the alkyl peroxy radicals to be well structured and observable using cavity ringdown spectroscopy (CRDS).Historically this IR transition has been viewed as difficult to study because of its small oscillator strength (the cross-section σ is ≈ 2 × 10 −21 cm −2 for the corresponding transition 13 in HO 2 ·) and the near IR spectral region (≈ 7000 − 8000 cm −1 ) in which the transition is located. However CRDS is a powerful technique 14-16 for dealing with these difficulties. T...

We report high-sensitivity detection of 2,4,6-trinitrotoluene (TNT) by using laser photoacoustic spectroscopy where the laser radiation is obtained from a continuous-wave room temperature high-power quantum cascade laser in an external grating cavity geometry. The external grating cavity quantum cascade laser is continuously tunable over Ϸ400 nm around 7.3 m and produces a maximum continuouswave power of Ϸ200 mW. The IR spectroscopic signature of TNT is sufficiently different from that of nitroglycerine so that unambiguous detection of TNT without false positives from traces of nitroglycerine is possible. We also report the results of spectroscopy of acetylene in the 7.3-m region to demonstrate continuous tunability of the IR source.quantum cascade lasers ͉ high-power lasers ͉ continuous-wave operation ͉ room temperature operation ͉ TNT detection D etection of illegally transported explosives has become important since the global rise in terrorism subsequent to the events of September 11, 2001. Although not a choice of suicide bombers, 2,4,6-trinitrotoluene (TNT) is a potent explosive for which techniques for detection on a person's body or in one's baggage is considered important for assuring safety of airports and air travel. As with detection of other similar compounds, such as chemical warfare agents, any detection scheme that claims to detect these targets must exhibit acceptable receiver operational characteristic (ROC) that assures detection at very low levels without an unacceptable level of false alarms (1, 2). The molecular mass of TNT (C 7 H 5 N 3 O 6 ) is almost exactly identical to the molecular mass of nitroglycerine (C 3 H 5 N 3 O 9 ) even though the chemical compositions of the two molecules are very different (TNT, 227.131 Da vs. nitroglycerine, 227.0872 Da). The nearly same molecular masses often lead to problems for unambiguous detection of TNT using techniques that rely on measuring the molecular mass of the species. On the other hand, the differences in the chemical structure between TNT and nitroglycerine lead to noticeably different infrared (IR) absorption signatures (3), making it possible to distinguish between the two. However, the detection of TNT in vapor phase is hampered by its low vapor pressure of Ϸ2 ϫ 10 Ϫ4 torr at 25°C. In this work, we report on studies of detection of TNT by using room-temperature (RT) quantum cascade laser (QCL)-based photoacoustic spectroscopy (QCL-PAS). The high sensitivity afforded by laser-based photoacoustic spectroscopy (L-PAS) (4) shows that the vapor-phase detection of TNT at an ambient temperature of Ϸ25°C is possible.Previously, CO and CO 2 lasers have been used for photoacoustic (PA) spectroscopic detection (3, 5) of vapors of explosives. However, both of these laser sources are step tunable, and neither of the lasers is able to access the strong absorption features of TNT that lie in the 6.0-7.5 m region. Quantum cascade lasers (QCLs), with their continuous tunability, should be the right sources for the detection of TNT and other species that do...

The laser induced fluoresence excitation spectrum for the A 2 A 1 ↔ X 2 E transition of the methoxy radical has been reinvestigated. An extensive set of vibrational levels has been assigned with the aid of increased vibrational and rotational cooling. Many of these vibrational assignments are confirmed by rotational analysis of bands involving both the symmetric and asymmetric fundamentals of the A state as well as vibrations containing two quanta of the e modes. Although parts of the vibrational structure have been assigned previously, several discrepancies are identified and corrected. Vibrational frequencies have been obtained for all the modes in the A 2 A 1 state of the molecule. The Fermi resonance that exists between ν 3 and ν 2 has been investigated and interaction constants describing it have been obtained.

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.

We report the sub-parts-per-billion-level detection of NO2 using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of Ϸ300 mW tunable over Ϸ350 nm centered at 6.25 m. We demonstrate minimum detection level of Ϸ0.5 parts per billion of NO2 in the presence of humidified air.IR lasers ͉ NO2 spectroscopy ͉ photoacoustic spectroscopy ͉ sub-parts-per-billion detection of gases R eal-time trace-level gas detection is an area of fast growth with applications in such diverse fields as medical diagnostics, process control, national security, and environmental airquality monitoring. Techniques based on the measurement of optical absorption using tunable laser sources are attractive detection methods because of their intrinsic sensitivity and selectivity, i.e., an ability for discriminating against interferents (1, 2). The most recent advances, intercomparison of spectroscopic techniques, and status of available laser sources are summarized in a recent review (3). Furthermore, photoacoustic (PA) spectroscopy is seen to be well suited for optical absorption measurements because it combines high sensitivity with the ruggedness needed for field-deployable instrumentation. However, laser PA spectroscopy (L-PAS) requires multihundredmilliwatt-level laser sources to achieve a sub-parts-per-billion (sub-ppb) level of sensitivity for many of the environmentally and industrially important gases. As a result, heretofore most of the sensitive L-PAS detection schemes have used continuouswave (CW) molecular gas lasers (4). Results and DiscussionHigh-Power Continuously Tunable Laser Source. The tuning characteristics of our high-power single-frequency source are seen in Fig. 1, which shows the single-mode laser power available from the CW room temperature (RT) external grating cavity (EGC) quantum cascade laser (QCL) (near 6.3 m) at several grating settings. Fig. 1 also shows the overall tuning of the output that covers Ͼ350 nm with CW RT laser power in excess of Ϸ200 mW with maximum CW RT laser power of 300 mW near the center of the tuning curve. Details are given in Materials and Methods.PA Spectroscopy of NO2. We have used the broadly and continuously tunable, high-power CW RT EGC-QCL source for PA spectroscopy of NO 2 , which exhibits strong absorption features near Ϸ1,600 cm Ϫ1 . NO 2 is a smog and particulate matter precursor and one of the key pollutants that is routinely monitored. Ambient concentrations of NO 2 typically are in single digits to tens of parts per billion (ppb) levels (5). The national ambient air quality standard (NAAQS) level for NO 2 is 53 ppb for arithmetic mean average (6). Consequently, an acceptable ambient NO 2 sensor should have single-digit ppb or, better, sub-ppb, sensitivity. Earlier, cryogenically cooled lead salt lasers (7) operat...