Lal A. Pinnaduwage

Sensitive detection of plastic explosives with self-assembled monolayer-coated microcantilevers

We report the detection of 10–30 parts-per-trillion levels of pentaerythritol tetranitrate and hexahydro-1,3,5-triazine within 20 s of exposure to a silicon microcantilever with its gold surface modified with a self-assembled monolayer of 4-mercaptobenzoic acid. These measurements correspond to a limit of detection of a few fg.

Explosives: A microsensor for trinitrotoluene vapour

Sensing devices designed to detect explosive vapours are bulky, expensive and in need of technological improvement — dogs remain the most effective detectors in the fight against terrorism and in the removal of land-mines. Here we demonstrate the deflagration of trinitrotoluene (TNT) in a small localized explosion on an uncoated piezoresistive microcantilever. This explosive-vapour sensor, which has a detection capability that is comparable to that of a dog, should enable extremely sensitive, miniature detection devices to be used on a large scale.

Detection of trinitrotoluene via deflagration on a microcantilever

We describe in detail the detection of deflagration of trinitrotoluene (TNT) deposited on a piezoresistive microcantilever and point out its possible use for explosive-vapor detection. The deflagration of TNT causes the cantilever to bend (due to released heat) and its resonance frequency to shift (due to mass unloading). Explosive vapors provide unique responses that are absent for “interferences” such as water or alcohol vapors. The proposed sensor makes possible a sensitive, miniature explosives detection device that may be deployed in large numbers. The minimum amount of TNT detected on the cantilever depends on the cantilever dimensions and was ≈50 pg for the batch of cantilevers used.

Moore's law in homeland defense: an integrated sensor platform based on silicon microcantilevers

An urgent need exists for the development of inexpensive, highly selective, and extremely sensitive sensors to help combat terrorism. If such sensors can be made miniature, they could be deployed in virtually any situation. Terrorists have a wide variety of potential agents and delivery means to choose from for chemical, biological, radiological, or explosive attacks. Detecting terrorist weapons has become a complex and expensive endeavor, because a multitude of sensor platforms is currently needed to detect the various types of threats. The ability to mass produce and cost effectively deploy a single type of sensor that can detect a wide range of threats is essential in winning the war on terrorism. Silicon-based microelectromechanical sensors (MEMS) represent an ideal sensor platform for combating terrorism because these miniature sensors are inexpensive and can be deployed almost anywhere. Recently, the high sensitivity of MEMS-based microcantilever sensors has been demonstrated in the detection of a variety of threats. Therefore, the critical requirements for a single, miniature sensor platform have been met and the realization of an integrated, widely deployable MEMS sensor could be near.

Electron attachment to excited states of silane: Implications for plasma processing discharges

Observation of enhanced negative-ion formation in ArF–excimer–laser irradiated silane was reported in a recent paper [L. A. Pinnaduwage, M. Z. Martin, and L. G. Christophorou, Appl. Phys. Lett. 65, 2571 (1994)]. In that paper, preliminary evidence was presented to show that highly excited electronic states of silane or its photofragments could be responsible for the observed enhanced negative-ion formation. In the present paper, we report evidence, obtained using a new experimental technique, that the electron attaching species are high-Rydberg (HR) states of silane indirectly populated via laser irradiation and show that an absolute lower bound for the corresponding electron attachment rate constant is ∼4×10−7 cm3 s−1. The initial capture of the electron by the HR states is likely to be a diabatic process and the large polarizabilities associated with the HR states appear to be responsible for the observed large electron attachment rate constants. We also measured electron attachment to thermally excited...

Optically enhanced electron attachment to thiophenol

Enhanced electron attachment to KrF excimer laser irradiated thiophenol (C6H5SH) molecules has been studied in a electron swarm experiment using nitrogen as the buffer gas. Two distinct electron attachment processes were found to be responsible for the observed large enhancement in electron attachment under different experimental conditions. One enhanced electron attachment process occurred immediately (within a few μs) after laser irradiation and is shown to be due to dissociative electron attachment to electronically excited thiophenol molecules in their first excited triplet state produced indirectly via excited singlet states reached by excimer laser irradiation. At low mean electron energies (∼0.1 eV), up to 5 orders of magnitude enhancement in electron attachment has been observed for the triplet state compared to the ground electronic state. This enhanced electron attachment decreased with (i) increasing nitrogen pressure due to quenching of the first excited singlet state of thiophenol (precursor ...

Enhanced electron attachment to superexcited states of saturated tertiary amines

Electron attachment measurements on excimer–laser‐excited superexcited states (SES) of saturated amine compounds, and in particular on triethylamine (TEA), have been carried out employing a new experimental technique. A rate equation analysis based on a proposed model shows that the electron attachment rate constant for the SES is several orders of magnitude larger than that for the ground electronic state. The proposed mechanism for electron attachment to SES involves the capture of a near‐zero‐energy electron—(produced by the same laser pulse that produces the SES)—by a superexcited molecule to form a transient parent anion which subsequently dissociates producing a stable fragment anion. The similarity of the above mechanism to an electron‐excited Feshbach resonance is indicated and a scheme for the identification of molecular systems that can be excited (via resonance‐enhanced multiphoton excitation) to SES is outlined.

VERIFICATION OF H- FORMATION IN ULTRAVIOLET-LASER-IRRADIATED HYDROGEN : IMPLICATIONS FOR NEGATIVE ION AND NEUTRAL BEAM TECHNOLOGIES

Photodetachment and ion mobility measurements are reported confirming the efficient H− formation in UV‐laser‐irradiated H2 reported earlier [L. A. Pinnaduwage and L. G. Christophorou, Phys. Rev. Lett. 70, 754 (1993)]. The implications of the efficienct H− formation in UV‐laser‐irradiated H2 (and other types of negative ions in UV‐laser‐irradiated gases) for negative ion and neutral particle beam technologies are discussed. Also, the possible contribution to H− formation in H2 discharge sources from electron attachment to high‐lying electronically excited states of H2 is indicated.

Novel technique for real-time monitoring of electron attachment to laser-excited molecules

We report a new experimental technique that is capable of monitoring electron attachment to laser‐excited molecules in real time; the time resolution is limited only by the time constant of the detection circuit and was ∼100 ps for the experiments reported here. This technique provides information on the lifetime of the excited states responsible for electron attachment, and also allows determination of electron attachment cross sections involved. Results on dissociative electron attachment to ArF‐excimer‐laser‐irradiated NO are reported: Electron attachment occurred to the A 2Σ+(ν=3) state populated via the absorption of a single photon, and to highly excited states populated via two‐photon absorption; the cross section for low‐energy electron attachment to the A 2Σ+(ν=3) state was ∼3 orders of magnitude larger compared to that for the A 2Σ+(ν=0). Decay of the electrons over the ∼200 ns lifetime of the A 2Σ+(ν=3) state was directly monitored. Negative‐ion formation that occurred via the A 2Σ+(ν=3) state ...

Enhanced electron attachment to Rydberg states in molecular hydrogen volume discharges

We review recent studies on negative ion formation and studies in other areas that are relevant to the role of high-Rydberg states of H2 and H3 in hydrogen negative ion sources. Possible mechanisms for the formation of these excited states are discussed, including the formation of long-lived superexcited (core-excited) Rydberg states. Experimental evidence for negative ion formation via electron attachment to core-excited Rydberg states in a glow discharge apparatus is presented. An expression for the dissociative electron attachment rate constant for Rydberg molecules is derived based on electron capture by a Rydberg molecule due to polarization interaction.