S. Venugopal Rao

Femtosecond and nanosecond LIBS studies of nitroimidazoles: correlation between molecular structure and LIBS data

Co-reporter:E. Nageswara Rao, P. Mathi, S. Abdul Kalam, S. Sreedhar, Ajay K. Singh, B. N. Jagatap and S. Venugopal Rao

Journal of Analytical Atomic Spectrometry 2016 vol. 31(Issue 3) pp:737-750

Publication Date(Web):23 Dec 2015

DOI:10.1039/C5JA00445D

In the present study, seven novel explosive molecules (nitroimidazoles) have been investigated for laser induced breakdown (LIB) spectral signatures of molecular and atomic species in air and argon atmospheres utilizing both femtosecond (fs) and nanosecond (ns) laser pulse excitation. The molecular emissions were observed to be stronger in the fs spectra whereas atomic emissions were prominent in the ns spectra recorded in both air and argon atmospheres. The C2 Swan band was strongest in an argon atmosphere while the CN violet band was strongest in ambient environment. The LIB spectra were analysed for understanding (a) the influence of molecular structure i.e. type of bonds (C–C, CC, C–N and CN) on atomic (C, H, N and O) and molecular (CN, C2 and NH) emissions, (b) effect of surrounding atmosphere on the fs and ns LIB spectra, (c) correlation between stoichiometric and intensity ratios of molecular as well as atomic species, (d) effect of the number of substituents and their position in the ring on the fragmentation pathways and (e) correlation between oxygen balance and LIB spectra. Furthermore, time resolved spectroscopic studies of the plasma induced by fs and ns laser pulses have been carried out to understand the temporal evolution and possible reaction mechanisms of various molecular species. An attempt has been made to correlate the spectral emission with the chemical structure for this series of energetic materials. Our detailed studies and analyses clearly suggest that the atomization/fragmentation ratio could serve as a performance metric for high energy materials.

Surface enhanced fluorescence from corroles and SERS studies of explosives using copper nanostructures

Co-reporter:Syed Hamad, G. Krishna Podagatlapalli, Md. Ahamad Mohiddon, S. Venugopal Rao

Chemical Physics Letters 2015 Volume 621() pp:171-176

Publication Date(Web):4 February 2015

DOI:10.1016/j.cplett.2015.01.006

Highlights

•: Ps laser induced surface nanostructures fabricated using ablation in chloroform.
•: Complex Cu nanoparticles also fabricated using ps ablation in chloroform.
•: Surface enhanced fluorescence (SEF) studies were performed.
•: We have obtained huge enhancements (10⁵) in SEF studies.
•: SERS measurements of secondary explosive molecules of ANTA and Fox-7 were performed.

Sterically demanding zinc(II) phthalocyanines: synthesis, optical, electrochemical, nonlinear optical, excited state dynamics studies

Co-reporter:Debasis Swain, Radhakant Singh, Varun Kumar Singh, Narra Vamsi Krishna, Lingamallu Giribabu and Soma Venugopal Rao

Journal of Materials Chemistry A 2014 vol. 2(Issue 9) pp:1711-1722

Publication Date(Web):03 Dec 2013

DOI:10.1039/C3TC31640H

Two novel sterically demanding zinc phthalocyanines were synthesized and characterized. Their optical and electrochemical properties were investigated in detail. The emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.2–0.3 while time-resolved fluorescence data revealed radiative lifetimes of typically few ns. Nonlinear optical (NLO) properties were also evaluated at wavelengths of 600, 640, 680, and 800 nm using picosecond (∼1.5 ps) pulses and further studies with femtosecond (∼140 fs) pulses at 800 and 780 nm were also performed. Two-photon absorption (2PA) and saturable absorption (SA) were the dominant nonlinear absorption mechanisms observed with ps/fs excitation at different wavelengths in the visible spectral region. NLO coefficients were extracted from ps/fs closed and open aperture Z-scan measurements. Large two-photon absorption cross-sections of ∼14000 GM and n2 values in the range of 1–7 × 10−16 cm2 W−1 were retrieved for these molecules from the ps Z-scan data. The excited state decay dynamics were investigated using degenerate pump-probe experiments with ∼70 fs pulses near 600 nm. Double exponential fits of the pump-probe data suggested two decay times for both molecules investigated.

Effect of oblique incidence on silver nanomaterials fabricated in water via ultrafast laser ablation for photonics and explosives detection

Co-reporter:G. Krishna Podagatlapalli, Syed Hamad, Md. Ahamad Mohiddon, S. Venugopal Rao

Applied Surface Science 2014 Volume 303() pp:217-232

Publication Date(Web):1 June 2014

DOI:10.1016/j.apsusc.2014.02.152

Highlights

•: Effect of non-zero angle of incidence on ps ablation of Ag investigated.
•: Ag colloids were evaluated by TEM, UV–vis absorption spectra and fs-DFWM.
•: 30° incident angle provided Ag NPs of small size with higher yields.
•: FESEM, AFM, Raman data revealed the fabrication of Ag nanostructures.
•: Utility of Ag nanostructures surfaces for multiple SERS studies demonstrated.

Strong two-photon absorption properties and ultrafast pump-probe studies of novel porphyrin derivatives

Co-reporter:Debasis Swain, Anup Rana, Pradeepta K. Panda, S. Venugopal Rao

Chemical Physics Letters 2014 s 610–611() pp: 310-315

Publication Date(Web):

DOI:10.1016/j.cplett.2014.07.013

Experimental and theoretical studies on the structure and vibrational properties of nitropyrazoles

Co-reporter:E. Nageswara Rao, P. Ravi, Surya P. Tewari, S. Venugopal Rao

Journal of Molecular Structure 2013 Volume 1043() pp:121-131

Publication Date(Web):5 July 2013

DOI:10.1016/j.molstruc.2013.03.038

•Pyrazoles as models of simple aromatic systems.•Observed vibrational frequencies with the calculated at the B3LYP/aug-cc-pVDZ was superior to B3LYP/311++G∗∗ basis set.•Calculated geometries of pyrazoles are in excellent agreement with the average experimental geometries.•Differences in the bond distances and angles are confined to the twist of the nitro group.We report a theoretical and experimental study on the structure and vibrational properties of pyrazole and its mononitropyrazoles. The infrared (IR) and Raman spectra of pyrazole, N-nitropyrazole, 3-nitropyrazole and 4-nitropyrazole have been recorded in the solid state. To interpret the experimental data, ab initio computations of the vibrational frequencies were carried out using the Gaussian 03 program following the full optimizations at the HF/6-311++G(d,p), B3P86/6-311++G(d,p), B3LYP/6-311++G(d,p) and B3LYP/aug-cc-pVDZ levels. The combined use of experiments and computations allowed a firm assignment of the majority of observed bands for all compounds. The calculated stretching frequencies have been found to be in good agreement with the experimental frequencies. However, the fundamental vibrational frequencies of nitropyrazoles calculated at the B3LYP/aug-cc-pVDZ level are superior compared with those values that are obtained at the HF/311++G(d,p), B3P86/311++G(d,p) and B3LYP/311++G(d,p) levels of theory. The differences in the bond distances and bond angles are confined to the twist of the nitro group that present the greatest deviation from planarity in molecules.

Molecular formation dynamics of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one, 1,3,5-trinitroperhydro-1,3,5-triazine, and 2,4,6-trinitrotoluene in air, nitrogen, and argon atmospheres studied using femtosecond laser induced breakdown spectroscopy

Co-reporter:Sunku Sreedhar, E. Nageswara Rao, G. Manoj Kumar, Surya P. Tewari, S. Venugopal Rao

Spectrochimica Acta Part B: Atomic Spectroscopy 2013 Volume 87() pp:121-129

Publication Date(Web):1 September 2013

DOI:10.1016/j.sab.2013.05.006

•LIBS studies of NTO, RDX, and TNT in nitrogen, air, and argon were performed using fs pulses.•Decay constants of C, CN in three atmospheres were recorded systematically.•Different molecular to elemental ratios were investigated in detail.•CN/C and CN/N ratios were observed to be prominent in nitrogen and air atmospheres.Femtosecond laser induced breakdown spectroscopic (LIBS) studies were performed on three high energy materials namely 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and 2,4,6-trinitrotoluene (TNT). LIBS spectral features were obtained for these samples in three different atmospheres i.e. air, nitrogen, and argon. Different molecular to elemental ratios in these three atmospheres were investigated in detail. CN/C and CN/N ratios were observed to be prominent in nitrogen and air atmospheres. We attempt to elucidate the role of several reactions involving CN molecular formation in connection with discrepancies obtained in the measured ratios. The complete temporal dynamics of atomic C (247.82 nm) and CN (388.20 nm) molecular species in three different atmospheres are elaborated. The decay rates of C peak were found to be longest (96 ns–121 ns) in argon atmosphere for all the samples. The decay rates of CN peak (388.2 nm) were longer (161 ns–364 ns) in nitrogen compared to air and argon atmospheres. We also attempt to explicate the decay mechanisms with respect to the molecular species formation dynamics in different atmospheres.

Femtosecond and nanosecond laser induced breakdown spectroscopic studies of NTO, HMX, and RDX

Co-reporter:Sreedhar Sunku, Manoj Kumar Gundawar, Ashwin Kumar Myakalwar, P. Prem Kiran, Surya P. Tewari, S. Venugopal Rao

Spectrochimica Acta Part B: Atomic Spectroscopy 2013 s 79–80() pp: 31-38

Publication Date(Web):

DOI:10.1016/j.sab.2012.11.002

Dispersion studies of optical nonlinearity and excited state dynamics in cyclo[4]naphthobipyrroles

Co-reporter:Debasis Swain, P.T. Anusha, Tridib Sarma, Pradeepta K. Panda, S. Venugopal Rao

Chemical Physics Letters 2013 580() pp: 73-77

Publication Date(Web):

DOI:10.1016/j.cplett.2013.06.049

Ultrafast Excited-State Dynamics and Dispersion Studies of Third-Order Optical Nonlinearities in Novel Corroles

Co-reporter:P. T. Anusha, Debasis Swain, S. Hamad, L. Giribabu, T. Shuvan Prashant, Surya P. Tewari, and S. Venugopal Rao

The Journal of Physical Chemistry C 2012 Volume 116(Issue 33) pp:17828-17837

Publication Date(Web):July 19, 2012

DOI:10.1021/jp305497b

Ultrafast nonlinear optical (NLO) properties of four novel Corroles in the visible spectral range (660–800 nm) were evaluated using picosecond Z-scan technique. Ultrafast excited state dynamics have also been appraised with picosecond (ps) and femtosecond (fs) degenerate pump–probe techniques using excitation wavelengths of 800 and 600 nm, respectively. The excitation by 800 nm photons resulted in two-photon absorption at adequately high peak intensities, thereby facilitating the access to higher excited states (Sn). The nonradiative relaxation mechanisms from these states, reflected in the pump–probe data, consisted of double-exponential decay with a slow component in the range of 54–277 ps and faster component in the range of 2.0 to 2.5 ps. When excited with 600 nm photons (unfocused), photoinduced absorption was observed with the first excited state S1 being populated, and as a consequence single decay was observed in the data of all molecules studied. These retrieved lifetimes were analogous to those obtained with picosecond pump–probe data. The long lifetime is attributed to nonradiative decay from the S1 state with possible contribution from triplet states, whereas the shorter lifetime is attributed to the internal conversion (S2 to S1*), followed by vibrational relaxation (S1* to S1) processes. Time-resolved fluorescence lifetime measurements revealed the magnitude of radiative lifetimes to be in the nanosecond regime. NLO coefficients were evaluated from the Z-scan data at wavelengths of 660, 680, 700, 740, and 800 nm. Large two-photon absorption coefficients (β)/cross-sections (σ2) at 740 nm/680 nm were recorded for these molecules, making them apposite for applications such as two-photon induced photodynamic therapy and lithography. Figure of merit, T, was <1 for all molecules at 740 and 800 nm, suggesting that these molecules find use in photonic device applications.

Picosecond nonlinear optical studies of unsymmetrical alkyl and alkoxy phthalocyanines

Co-reporter:P.T. Anusha, P. Silviya Reeta, L. Giribabu, Surya P. Tewari, S. Venugopal Rao

Materials Letters 2010 Volume 64(Issue 17) pp:1915-1917

Publication Date(Web):15 September 2010

DOI:10.1016/j.matlet.2010.06.004

We present our results on picosecond nonlinearities in unsymmetrical alkyl and alkoxy phthalocyanines, studied using the Z-scan technique. Both open and closed aperture Z-scan curves were recorded and nonlinear coefficients were extracted. Solvent contribution to the nonlinearity was identified. We recorded large, off-resonant second hyperpolarizabilities (γ) at 800 nm with estimated values of ∼ 6.5 × 10−31 esu and ∼ 8.9 × 10−31 esu for unsymmetrical alkyl and alkoxy phthalocyanines, respectively. Evaluated figures of merit indicate that these are potential molecules for photonic applications.

Ultrafast nonlinear optical properties of alkyl phthalocyanines investigated using degenerate four-wave mixing technique

Co-reporter:R. Sai Santosh Kumar, S. Venugopal Rao, L. Giribabu, D. Narayana Rao

Optical Materials 2009 Volume 31(Issue 6) pp:1042-1047

Publication Date(Web):April 2009

DOI:10.1016/j.optmat.2008.11.018

We present our results on the investigation of ultrafast nonlinear optical properties including the time response of 2(3), 9(10), 16(17), 23(24) tetra tert-butyl phthalocyanine (pc1) and 2(3), 9(10), 16(17), 23(24) tetra tert-butyl Zinc phthalocyanine (pc2) studied using degenerate four-wave mixing technique at a wavelength of 800 nm with 100 fs pulses. We recorded large off-resonant second hyperpolarizabilities (γ) with estimated values of (4.27 ± 0.43) × 10−31 esu and (4.32 ± 0.43) × 10−31 esu for pc1 and pc2, respectively, with ultrafast nonlinear optical response in the femtosecond domain. We also estimated the figures of merit for photonic switching applications for both one-photon and three-photon loss mechanisms. The performance of these molecules vis-à-vis other molecules, in general, and phthalocyanines, in particular, is discussed.

Optical studies of two dimensional gratings in fused silica, GE 124, and Foturan™ glasses fabricated using femtosecond laser pulses

Co-reporter:K.C. Vishnubhatla, S. Venugopal Rao, R. Sai Santosh Kumar, Maurizio Ferrari, D. Narayana Rao

Optics Communications 2009 Volume 282(Issue 23) pp:4537-4542

Publication Date(Web):1 December 2009

DOI:10.1016/j.optcom.2009.08.041

Herein we present results on the femtosecond laser direct writing and optical characterization studies of two dimensional gratings in fused silica, GE 124, and Foturan™ glasses. Varieties of structures were achieved with varying input energy and spatial orientation of the samples. Various characterization techniques including fluorescence spectroscopy, micro-Raman spectroscopy, and laser confocal microscopy were employed to analyze the structural and physical modifications at the focal volume resulting in change of refractive index. Diffraction efficiencies of ∼9–12% were observed from the grating structures. A broad-band emission was observed in the laser-modified region of the Foturan glass. The obtained results are analyzed in the light of recent work in similar glasses and exploring the applications of such structures in the fields of photonics.

Femtosecond nonlinear optical properties of alkoxy phthalocyanines at 800 nm studied using Z-Scan technique

Co-reporter:N. Venkatram, D. Narayana Rao, L. Giribabu, S. Venugopal Rao

Chemical Physics Letters 2008 Volume 464(4–6) pp:211-215

Publication Date(Web):23 October 2008

DOI:10.1016/j.cplett.2008.09.029

Abstract

We report our results on the nonlinear absorption, and its switching behavior, of two alkoxy phthalocyanines (a) 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine (Pc1) and (b) 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) (Pc2) studied at a wavelength of 800 nm with ∼100 fs pulses using the standard Z-scan technique. Both the molecules possessed negative nonlinear refractive index (n₂) as revealed by signature of the closed aperture data. Magnitudes of the n₂ evaluated were ∼1.26 × 10⁻¹³ esu for Pc1 and ∼1.68 × 10⁻¹³ esu for Pc2. Nonlinear absorption data of Pc1 obtained at a concentration of 5 × 10⁻⁴M demonstrated complex behavior with switching from reverse saturable absorption (RSA) within saturable absorption (SA) at low peak intensities to SA in RSA at higher peak intensities. Pc2 data recorded at similar concentration exhibited saturable absorption type of behavior at lower peak intensities. The nonlinear absorption at higher intensities was again multifarious with switching over from SA to RSA. We discuss the complicated behavior of the nonlinear absorption and nonlinear optical performance of these molecules for their potential applications.

Large third-order optical nonlinearity and optical limiting in symmetric and unsymmetrical phthalocyanines studied using Z-scan

Co-reporter:S.J. Mathews, S. Chaitanya Kumar, L. Giribabu, S. Venugopal Rao

Optics Communications 2007 Volume 280(Issue 1) pp:206-212

Publication Date(Web):1 December 2007

DOI:10.1016/j.optcom.2007.08.022

The third-order nonlinear optical and optical limiting properties of 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine, 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine zinc(II) (symmetric) and 2(3)-(butane-1,4-dioic acid)-9(10),16(17),23(24)-tri tert-butyl phthalocyanine zinc(II) (unsymmetrical) have been investigated using a continuous wave laser at 633 nm. We have employed the Z-scan technique to evaluate the sign and magnitude of nonlinear refractive index and the nonlinear absorption coefficient. Optical limiting based on nonlinear refraction was performed and limiting thresholds were estimated for all the three samples. The magnitude of the third-order nonlinearities measured were one of the highest reported in the cw regime till date. Unsymmetrical phthalocyanine had better limiting characteristics and its nonlinear coefficients were comparatively high. Results of the nonlinear optical studies performed with these organic dyes doped in Poly(methylmethacrylate) are also presented. Our studies indicate that these phthalocyanines are potential candidates for optical limiting applications in low power cw regime.

Sterically demanding zinc(II) phthalocyanines: synthesis, optical, electrochemical, nonlinear optical, excited state dynamics studies

Co-reporter:Debasis Swain, Radhakant Singh, Varun Kumar Singh, Narra Vamsi Krishna, Lingamallu Giribabu and Soma Venugopal Rao

Journal of Materials Chemistry A 2014 - vol. 2(Issue 9) pp:NaN1722-1722

Publication Date(Web):2013/12/03

DOI:10.1039/C3TC31640H

Two novel sterically demanding zinc phthalocyanines were synthesized and characterized. Their optical and electrochemical properties were investigated in detail. The emission spectra were recorded in different solvents and the fluorescence yields obtained were in the range of 0.2–0.3 while time-resolved fluorescence data revealed radiative lifetimes of typically few ns. Nonlinear optical (NLO) properties were also evaluated at wavelengths of 600, 640, 680, and 800 nm using picosecond (∼1.5 ps) pulses and further studies with femtosecond (∼140 fs) pulses at 800 and 780 nm were also performed. Two-photon absorption (2PA) and saturable absorption (SA) were the dominant nonlinear absorption mechanisms observed with ps/fs excitation at different wavelengths in the visible spectral region. NLO coefficients were extracted from ps/fs closed and open aperture Z-scan measurements. Large two-photon absorption cross-sections of ∼14000 GM and n2 values in the range of 1–7 × 10−16 cm2 W−1 were retrieved for these molecules from the ps Z-scan data. The excited state decay dynamics were investigated using degenerate pump-probe experiments with ∼70 fs pulses near 600 nm. Double exponential fits of the pump-probe data suggested two decay times for both molecules investigated.

Femtosecond and nanosecond LIBS studies of nitroimidazoles: correlation between molecular structure and LIBS data

Co-reporter:E. Nageswara Rao, P. Mathi, S. Abdul Kalam, S. Sreedhar, Ajay K. Singh, B. N. Jagatap and S. Venugopal Rao

Journal of Analytical Atomic Spectrometry 2016 - vol. 31(Issue 3) pp:NaN750-750

Publication Date(Web):2015/12/23

DOI:10.1039/C5JA00445D

In the present study, seven novel explosive molecules (nitroimidazoles) have been investigated for laser induced breakdown (LIB) spectral signatures of molecular and atomic species in air and argon atmospheres utilizing both femtosecond (fs) and nanosecond (ns) laser pulse excitation. The molecular emissions were observed to be stronger in the fs spectra whereas atomic emissions were prominent in the ns spectra recorded in both air and argon atmospheres. The C2 Swan band was strongest in an argon atmosphere while the CN violet band was strongest in ambient environment. The LIB spectra were analysed for understanding (a) the influence of molecular structure i.e. type of bonds (C–C, CC, C–N and CN) on atomic (C, H, N and O) and molecular (CN, C2 and NH) emissions, (b) effect of surrounding atmosphere on the fs and ns LIB spectra, (c) correlation between stoichiometric and intensity ratios of molecular as well as atomic species, (d) effect of the number of substituents and their position in the ring on the fragmentation pathways and (e) correlation between oxygen balance and LIB spectra. Furthermore, time resolved spectroscopic studies of the plasma induced by fs and ns laser pulses have been carried out to understand the temporal evolution and possible reaction mechanisms of various molecular species. An attempt has been made to correlate the spectral emission with the chemical structure for this series of energetic materials. Our detailed studies and analyses clearly suggest that the atomization/fragmentation ratio could serve as a performance metric for high energy materials.

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