Laser-induced breakdown spectroscopy of ammonia gas with resonant vibrational excitation

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analysis, simultaneous multielement analysis, remote detection, and rapid in situ analysis. Hence, LIBS has widespread applications in geology, biomedicine, pharmaceuticals, environmental contamination, plasma chemistry, industrial monitoring, etc. [3]. In addition to the analyses used most often for solids and liquids, LIBS has also been used for gas analyses in applications of laser ignition [4], combustion diagnostics [5][6][7][8][9], and laser plasma chemistry [10,11]. However, to the authors' knowledge, LIBS for gas analyses has been reported only for molecules in natural states without resonant vibrational excitation. No studies about LIBS for molecules under resonant vibrational excitation have been reported. Therefore, this study will fill the current gap by investigating LIBS of gaseous molecules under resonant vibrational excitation. In addition, how the resonant vibrational excitation will affect the molecular breakdown/ionization process in various laser fields is interesting to be explored. Hence, this study will partially fill the gap of LIBS for gas detection under resonant vibrational excitation and provide inspirations for future research to explore how the resonant vibrational excitation will affect the molecular breakdown/ionization process as well as spectroscopy in various laser fields. Altered molecular ionization process from resonant vibrational excitation might result in composition variation of molecular fragments and play important roles in laser-assisted chemical reactions, such as laser-assisted growth of high-quality diamond [12] and laser-assisted low-temperature growth of gallium nitride [13].
In this study, laser-induced breakdown of gaseous ammonia (NH 3 ) molecules with no carbon dioxide (CO 2 ) laser beam irradiation and with CO 2 laser beam irradiation with wavelengths onand off-resonant (LIBS-RE-OFF) vibrational excitation of ammonia molecules were investigated and referred as LIBS, LIBS-RE-ON, and LIBS-RE-OFF, respectively. A wavelength-tunable, continuous wave (CW) CO 2 laser tuned to specific wavelengths of 9.219 and 10.591 µm, respectively, was used for on-and off-resonant vibrational excitation of ammonia molecules. A pulsed Nd:YAG laser (1064 nm, 15 ns) was used to break down the ammonia gas and induce plasmas. The plasma optical emission was collected and coupled into a spectrometer and a camera for spectroscopy and imaging measurements, respectively. Time-resolved spectra of the laser-induced plasmas were studied to explore the LIBS signal change with the gaseous molecule on-and off-resonant vibrational excitation. Fast imaging was used to investigate the spatial evolution of plasmas. Temporal evolution of the electron temperature and density of laser-induced plasmas were also studied to reveal the plasma property changes in LIBS, LIBS-RE-ON, and LIBS-RE-OFF, respectively.

Experiment setup
A schematic diagram of the experiment setup for LIBS of gaseous ammonia molecules in open air as well as with on-and off-resonant vibrational excitation of ammonia molecules is shown in Fig. 1. The gaseous ammonia was ejected vertically from a nozzle into the air with a flow rate of 80 standard cubic centimeters per minute (sccm). A flexible exhaust pipe was fixed 10 cm below the nozzle to draw away the excessive ammonia gas. A commercial Nd:YAG laser (Quantel DRL, 15 ns, 1064 nm) operated with a pulse energy of 630 mJ and a repetition rate of 5 Hz was horizontally focused by a plano-convex lens (diameter = 50.8 mm, focal length = 75 mm) to a spot size of 0.3 mm in diameter at a location 1.5 mm below the nozzle to induce breakdown of the gaseous ammonia. A wavelength-tunable CW CO 2 laser (PRC Inc., 9.2 ∼ 10.9 µm) beam was horizontally focused to a spot size of 1 mm in diameter by a zinc selenide (ZnSe) convex lens (focal length = 200 mm) and orthogonal to the Nd:YAG laser beam in the focal plane. The plasma emissions were collected by a light collector (Andor Technology, ME-OPT-0007) which was fixed in the horizontal plane with an angle of 45 degrees clockwise along the CO 2 laser beam direction. The plasma emission collected was coupled into a spectrometer through an optical fiber with a 200 µm core diameter. The spectrometer consists of a spectrograph (Andor Tech., Shamrock 303i) and an intensified charge-coupled device (ICCD) detector (Andor Tech., DH-712) attached to the exit focal plane of the spectrograph. The ICCD detector was externally triggered by the Nd:YAG laser for temporal synchronization. The gate delay and gate width of the ICCD detector are adjustable for temporal measurements. Three gratings of 150, 1200, and 2400 l/mm are included in the spectrometer. The 150 l/mm grating was used for all spectroscopy measurements in this study. An ICCD camera, comprised of a Nikon micro lens (105 mm, f/2.8 D) and an attached ICCD detector (Andor Tech., DH-734), was fixed in the horizontal plane with an angle of 135 degrees clockwise along the CO 2 laser beam direction and used for fast imaging. In this study, LIBS, LIBS-RE-ON, and LIBS-RE-OFF refer to ammonia gas without additional CO 2 laser beam irradiation, with the CO 2 laser beam irradiation at a wavelength of 9.219 µm (on-resonant vibrational excitation of the N-H wagging mode of ammonia molecules), and with the CO 2 laser beam irradiation at a wavelength of 10.591 µm (default CO 2 laser wavelength, off-resonant vibrational excitation of ammonia molecules), respectively. For both on-and off-resonant vibrational excitation, the output power of the CO 2 laser used was 33 watts (W). The relationship between the output wavelength and maximum power of the CO 2 laser is shown in Fig. 2(a). The absorption spectrum of the gaseous ammonia within the CO 2 laser wavelength tunable range was measured in open air at atmospheric pressure and is shown in Fig. 2(b).

Spectra and images of ammonia plasmas
Optical emission spectra and images of the laser-induced gaseous ammonia plasma at different delay times were measured by the spectrometer and ICCD camera, respectively, to investigate the spectral and spatial evolution of the plasmas. Figure 3 shows the plasma emission spectra measured at a delay time of 2 µs in LIBS (black line), LIBS-RE-ON (red line), and LIBS-RE-OFF (blue line). The spectral intensities in LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON spectral intensities. Enhanced atomic emission signals from both hydrogen and nitrogen were observed in LIBS-RE-ON.

Electron temperature and density of plasmas
The electron temperature and density of plasmas during the observation time window of 1 ∼ 10 µs were calculated to reveal the spectral characteristics. Figures 6(a) and 6(b) show the evolution of electron temperature and density, respectively, in LIBS (black squares), LIBS-RE-ON (red dots), and LIBS-RE-OFF (blue triangles). With the assumption of local thermodynamic equilibrium, the plasma temperature can be calculated from the relative intensity ratio of spectral lines from the same element ionization stage according to the Boltzmann distribution, using the equation of [3,14,15], where I i and λ i are the integrated line intensity and wavelength, respectively. Symbols of A i , g i , and E i are the transition probability, statistical weight, and excited energy for the upper level, respectively. Symbols of κ and T e are the Boltzmann constant and the electron temperature of the plasma, respectively.  [15]. The parameters of atomic lines used for temperature calculation are shown in Table 1 [16]. The emission line of H I 656.3 nm was used to obtain the electron density, N e , using the following expression given by Ashkenazy [17,18], N e = 8.02 × 10 12 (∆λ 1/2 /α 1/2 ) 3/2 cm −3 (2)  where ∆λ 1/2 is the full width at half maximum (FWHM) of the H I 656.3 nm line, and α 1/2 is half the width of the reduced Stark profiles in units of Å. The values of α 1/2 for the Balmer series can be found in several citations [19,20]. For more information about the electron temperature and density calculation of plasmas generated from laser-induced breakdown of gases, please refer to Refs. [21][22][23]. As shown in Fig. 6, the temporal evolution of the electron temperature of plasmas generated in LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas generated in LIBS-RE-ON possessed higher electron temperatures, such as those enhanced from ∼ 1.0×10 4 K in LIBS to ∼ 1.6×10 4 K in LIBS-RE-ON at the delay time of 1 µs, with a temperature enhancement of ∼ 64%. However, the temporal evolution of the electron density of plasmas generated in LIBS, LIBS-RE-OFF, and LIBS-RE-ON were close to each other except for a tiny increase in LIBS-RE-ON after a delay time of 3 µs. Therefore, the spectral signal enhancement in LIBS-RE-ON can primarily be attributed to the increase in the electron temperature of plasmas.
The electron temperature evolution in LIBS-RE-ON was similar to some previous studies about laser-induced plasma evolution in a high-temperature flame atmospheric environment, which showed that enhanced electron temperatures were observed in the high temperature atmospheric environment [24,25]. The enhanced electron temperature in LIBS-RE-ON is presumably due to the temperature increase in gaseous ammonia through absorption of the CO 2 laser energy at the resonant vibrational excitation wavelength of ammonia. Therefore, a real-time datalogging thermometer (Oakton, Temp-360, temperature range −210 to 1210°C) was used to measure the ammonia gas temperature under the conditions of without CO 2 laser irradiation, with CO 2 laser irradiation at a power of 33 W at the wavelength of 10.591 µm (off-resonant) and 9.219 µm (on-resonant), respectively. As shown in Fig. 7, the sensor of the thermometer was fixed 5 mm below the nozzle to measure the ammonia gas temperature before breakdown. Under this condition, the measured temperature of the ammonia gas without CO 2 laser irradiation was 20 ± 0.5°C. With CO 2 laser irradiation at a power of 33 W at the wavelengths of 10.591 µm (off-resonant) and 9.219 µm (on-resonant), the measured temperature of the ammonia gas was 27 ± 0.9°C and 180 ± 13°C, respectively. With resonant vibrational excitation of the ammonia molecules, the ammonia gas temperature increased by around 160°C before breakdown in this study. Hence, the enhancement of the electron temperature of the laser-induced plasma in LIBS-RE-ON was probably primarily due to the ammonia gas temperature increase before breakdown. In more detail, the gas temperature increase before breakdown in LIBS-RE-ON provides a higher temperature atmospheric environment in which the root mean square (RMS) velocity of electrons will be increased, which could result in enhanced avalanche breakdown process and hence enhanced electron temperature. Compared with the avalanche ionization mechanism in ns laser-induced breakdown process [26], the molecular breakdown/ionization mechanism differs primarily as multiphoton/tunneling ionization in ultrafast/intense laser fields [27][28][29]. Hence, under resonant vibrational excitation of gaseous molecules, different effects on the molecular breakdown/ionization in the aspects of plasma properties, composition of molecular fragments, and spectroscopy in an ultrafast/intense laser field could be expected, which will be interesting for future studies.

Summary
In this study, laser-induced breakdown of NH 3 under LIBS (no CO 2 laser beam irradiation), LIBS-RE-ON (with CO 2 laser beam irradiation at 9.219 µm, on-resonant vibrational excitation of ammonia), and LIBS-RE-OFF (with CO 2 laser beam irradiation at 10.591 µm, off-resonant vibrational excitation of ammonia) was studied in open air. The spectral and spatial evolution of plasmas generated in LIBS, LIBS-RE-ON, and LIBS-RE-OFF were investigated by spectral analyses and plasma imaging. The spectral and spatial evolution of plasmas generated in LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas generated in LIBS-RE-ON showed enhanced atomic emission signals and larger spatial plasma size. The electron temperature and density of plasmas were calculated to help explain the atomic emission signal enhancement in LIBS-RE-ON. The same as the tendency in the spectral evolution, plasma properties of electron temperature and electron density in LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas generated in LIBS-RE-ON showed enhanced electron temperature but almost the same electron density during the observation time window of 1 ∼ 10 µs, such as the electron temperature enhanced from ∼ 1.0×10 4 K in LIBS to ∼ 1.6×10 4 K (enhanced by ∼ 64%) in LIBS-RE-ON at the delay time of 1 µs. Therefore, the enhanced atomic emission signals of spectra in LIBS-RE-ON can be primarily attributed to the enhancement of electron temperatures. The electron temperature enhancement of the plasma in LIBS-RE-ON is probably due to the temperature increase in the gaseous ammonia through absorption of the CW CO 2 laser power at the on-resonant vibrational excitation wavelength of ammonia before laser breakdown of the gas. Enhanced emission signals in LIBS generally indicate improved sensitivity. This study could inspire future works on LIBS for gas detection with improved sensitivity probably by using ultrafast/intense laser-induced LIBS with resonant vibrational excitation of molecules.