DOI: 10.1007/s00340-003-1153-3 Appl. Phys. B 76, 691–697 (2003) Lasers and Optics Applied Physics B v.l. kasyutich ✉ c.s.e. bale c.e. canosa-mas c. pfrang s. vaughan r.p. wayne Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK Received: 4 February 2003/Revised version: 10 March 2003 Published online: 12 May 2003 • © Springer-Verlag 2003 ABSTRACT We present an application of cavity-enhanced ab- sorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO 2 ) and iodine monoxide (IO) radicals using a violet laser diode at λ = 404.278 nm. A noise-equivalent (1σ ≡ root-mean- square variation of the signal) fractional absorption for one optical pass of 4.5 × 10 −8 was demonstrated with a mirror re- flectivity of ∼ 0.99925, a cavity length of 0.22 m and a lock-in- amplifier time constant of 3 s. Noise-equivalent detection sen- sitivities towards nitrogen dioxide of 1.8 × 10 10 molecule cm −3 and towards the IO radical of 3.3 × 10 9 molecule cm −3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO 2 + NO → RO + NO 2 ). Measurement of oxygen-atom con- centrations was accomplished by determining the depletion of NO 2 in the reaction NO 2 + O → NO + O 2 . Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3 × 10 10 molecule cm −3 in the discharge-flow-tube ex- periments. PACS 42.55.Px; 42.62.Fi; 42.68.Ca; 82.30.Cf 1 Introduction Violet and blue laser diodes based on gallium ni- tride (GaN) or indium gallium nitride (InGaN) have recently become available commercially and emit in the spectral range of 395–445 nm with a power of 5– 30 mW [1, 2]. These laser diodes have been used in high-density optical data storage with a capacity of 22 GB for a 120-mm-diameter phase- change disk [3], in fluorescence spectroscopy of an indium atomic beam [4] at λ = 410 nm and of potassium atoms at λ = 404.5 nm and 404.8 nm [5], in wavelength-modulation spec- troscopy and two-tone frequency-modulation spectroscopy of potassium at λ = 404.8 nm and of lead at λ = 405.8 nm [6], in direct absorption spectroscopy of aluminium atoms at λ = 394.4 nm and 396.15 nm [7], in saturation spectroscopy on the 5 2 S 1/2 − 6 2 P 3/2 transition in rubidium at λ = 420 nm [8], in direct absorption spectroscopy of nitrogen dioxide (NO 2 ) at ✉ Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk λ = 390.13 nm [9], in continuous-wave (cw) cavity ring-down spectroscopy (CRDS) of NO 2 at λ ∼ 410 nm [10], in direct absorption spectroscopy of mercury at λ = 254 nm using sum- frequency generation (SFG) of violet (λ = 404 nm) and red (λ = 688 nm) laser diodes [11] and in direct absorption spec- troscopy of the hydroxyl radical at λ = 309 nm using SFG of violet (λ = 403.5 nm) and near-infrared distributed feedback (λ = 1320 nm) laser diodes [12]. A typical free-running vio- let or blue laser diode operates on a few modes separated by ∼ 0.05 nm [2] with a line width of a few hundreds of MHz [5]. However, single-mode operation can be achieved by means of judicious choices of temperature and current [6]. Alter- natively, stable single-mode generation of radiation can be obtained using an external grating configuration [13–16] with a typical line width of ∼ 1 MHz within an observation time of 50 ms. CRDS can provide high sensitivity for the detection of absorbing species: the technique is based on measurement of the time decay of the intensity of light trapped in an optical cavity containing the absorbing species [17]. The noise-equivalent absorption sensitivity is limited by the ac- curacy of the time-decay measurement, and reaches its op- timum values with increasing reflectivity of the mirrors and an increasing optical base path length for each pass through the absorber. An NO 2 noise-equivalent (1σ ≡ root-mean- square (RMS) variation of the concentration) detection limit of 9.9 × 10 9 molecule cm −3 (the absorption cross section is 7.18 ± 0.5 × 10 −19 cm 2 molecule −1 at λ = 410.4955 nm) was recently demonstrated by means of cw CRDS in a cavity consisting of two high-reflectivity spherical mirrors ( R = 0.999956, where R is the ratio of the reflected power to the incident power) mounted at a distance of 0.35-m apart [10]. This high sensitivity was achieved by arranging an isolation of the cavity from vibrations and by using external cavity laser diode, an optical isolator and an acousto-optic modula- tor. However, it is rather difficult to maintain isolation from vibration with the type of discharge-flow system used for gas- phase kinetic measurements of reactive intermediates [18, 19]. Extreme demands on the stability of the CRDS appara- tus can therefore make these systems complex and expensive to implement. Alternative methods can be adapted that still depend on many optical passes within a cavity. For example, rela- tively simple optical arrangements can provide sensitive ab- sorption measurements in cavity-enhanced absorption spec-