Multiphoton Ionization and Dissociation of Nitromethane Using Femtosecond Laser Pulses at 375 and 750 nm

Ultrafast Dynamics of Nitro-Nitrite Rearrangement and Dissociation in Nitromethane Cation

We report new insights into the ultrafast rearrangement and dissociation dynamics of nitromethane cation (NM⁺) using pump-probe measurements, electronic structure calculations, and ab initio molecular dynamics simulations. The "roaming" nitro-nitrite rearrangement (NNR) pathway involving large-amplitude atomic motion, which has been previously described for neutral nitromethane, is demonstrated for NM⁺. Excess energy resulting from initial population of the electronically excited D₂ state of NM⁺ upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO⁺. Both pump-probe measurements and molecular dynamics simulations are consistent with the completion of NNR within 500 fs of ionization with dissociation into NO⁺ and OCH₃ occurring ∼30 fs later. Pump-probe measurements indicate that NO⁺ formation is in competition with the direct dissociation of NM⁺ to CH₃⁺ and NO₂. Electronic structure calculations indicate that a strong D₀ → D₁ transition can be excited at 650 nm when the C-N bond is stretched from its equilibrium value (1.48 Å) to 1.88 Å. On the other hand, relaxation of the NM⁺ cation after ionization into D₀ occurs in less than 50 fs and results in observation of intact NM⁺. Direct dissociation of the equilibrium NM⁺ to produce NO₂⁺ and CH₃ can be induced with 650 nm excitation via a weakly allowed D₀ → D₂ transition.

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH₃-NO₂ structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO₂⁺ and CH₃⁺, rearrangement prior to dissociation accounts for fragmentation products including NO⁺, CH₂OH⁺, and CH₂NO⁺. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH₂⁺, H⁺, and NO₂²⁺.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

Discrimination of cancerous and healthy colon tissues: A new laser-based method

BACKGROUND

Femtosecond (fs) Laser Ionisation Mass Spectrometry (fs-LIMS) on colon tissues are described and investigated using ionization/fragmentation processes in details to present a new application in this study. Linear Time of Flight (L-TOF) mass analyzer was utilized to investigate paraffin-embedded human tissue in this study. The effect of fs laser intensity on the spectral characteristics was investigated and interpreted due to mass spectra obtained using 800 nm wavelength with 90 fs pulses at 1 kHz repetition rate.

OBJECTIVES

Mass spectra of tissues were recorded from L-TOF system and then analyzed by performing a statistical approach called Principal Component Analysis (PCA). The fs-LIMS method applied is proposed as a new and pioneering technology to analyze tissues using L-TOF system, as a human free fast and reliable intra-operative cancer diagnosis method for guiding surgeon to clean the edges of cancerous tissues to be applied during the surgical operation, for pathological examinations. Fs-LIMS provides some unique diagnosis opportunities to investigate biochemical characteristics of cancerous tissues leading to obtain sensitive, fast, and reliable results. The analysis of tissue is based on distribution of molecular ion (m/z) peaks in low mass region (<m/z 100) in mass spectra. Fs-LIMS provides a great data for identification of tissues (healthy and cancerous) in details. The effect of laser pulse was investigated in this study and also different types of lasers are utilized for various investigations from surgery to spectroscopy.

METHODS

The experimental setup mainly consists of an ultrafast (90 fs) laser system, a mass spectrometer, laser-tissue interaction/ablation chamber, data collection, and analysis system with windows based fast digital-storage oscilloscope, statistical application codes which have been developed by our group for analyses running under MATLAB software.

RESULTS

Paraffin embedded colon tissues have been distinguished from each other with statistical approaches using fs-LIMS based data as an alternative to histological and pathological examinations.

CONCLUSION

The fs-LIMS method provides a powerful novel approach to identify and analyze tissues. This promising method provides a fast and reliable (free of human mistakes) diagnosis and guidance for pathologists, surgeons, and patients during surgical operations, as well as increase the significance of mass spectrometric tissue analysis methods, especially with those capability of molecular identification of tissues. Lasers Surg. Med. © 2018 Wiley Periodicals, Inc.

A New Method for Investigation of Different Tissues Using Femtosecond Laser Mass Spectrometry

BACKGROUND

Femtosecond laser mass spectrometry (FLMS) has become an important tool for investigation of chemical and biological materials in many areas from medical to industrial.

OBJECTIVE

In medicine, the morphological examination of tissues is determined by performing pathological investigations under microscope. However, some novel improvements or developments must be performed for much faster diagnosis of the tissue during the operation when patient is under anesthesia.

METHODS

The information obtained from the tissue under the microscope remains very limited because it cannot reveal characteristics of the whole molecules. For this reason, some novel methods for analysis of tissues are important issues to be achieved. This process can be performed using FLMS much quicker than traditional techniques. The aim of this study is to develop a new procedure for interpretation of mass spectra obtained from different types of muscle tissues, such as lamb, bones, and beef obtained from the butcher.

RESULTS

The results obtained in this study are believed to open a new window for these kinds of applications for cancer diagnosis on human tissue studies, as a faster analysis technique to give some concrete contributions to pathological examinations. Both, principal component analysis statistical approach and FLMS technique offer a great opportunity to identify the biological materials from mass spectra. In conclusion, this present study interprets a great data from bone, beef, and lamb, which show that we can distinguish these different types of materials using FLMS data and statistical approaches.

CONCLUSIONS

Eventually, the experimental results obtained from our group studies present that these types of tissues can easily be distinguished using small m/q peaks in the lower region (m/z ≤100 amu) of the mass spectra by courtesy of FLMS.

Distinction of Structural Isomers of Benzenediamin and Difluorobenzene by Means of Chirped Femtosecond Laser Ionization Mass Spectrometry

Structural isomers of disubstituted benzenes are difficult to distinguish with most mass spectrometric methods. Consequently, conventional concepts for the distinction of isomers are based on coupling mass spectrometry with a chromatographic method. As an alternative approach, we propose the combination of femtosecond laser ionization with time-of-flight mass spectrometry (fs-LIMS). The possibility of systematic tailoring of fs-laser pulse shapes opens access to a multidimensional analytical technique capable of distinguishing structural isomers of the title molecules.

A mass spectrometric investigation of isomers of butane

RATIONALE

Butane is an important industrial chemical in which photo-processes are very important for the initiation of reactions. Recent advances in nanosecond pulsed laser technology have led to high laser intensities being available to researchers to enable these photo-processes to be studied in compounds such as butane.

METHODS

The photo-decomposition, dissociation and combustion mechanisms in the neutral butane molecule have been studied in detail, by investigating the multiphoton (MP) dissociative ionisation of its n- and i-isomers, using a time-of-flight mass spectrometer connected to a high power nanosecond laser system. The laser used was a Nd:Yag with a 5 ns pulse width operated at the fundamental wavelength (1064 nm) and the doubled and tripled wavelengths (532 nm and 355 nm). The fragmentation patterns for the isomers were determined for the three wavelengths as a function of laser intensity. Similar laser intensities of between 10(10) and 10(13) W/cm(2) were used at the three wavelengths: 1064, 532 and 355 nm.

RESULTS

The mass spectra of each isomer of the butane molecule display a very weak molecular ion and are dominated by fragment ion peaks. The degree of fragmentation increases as the laser intensity increases.

CONCLUSIONS

Depending on the wavelength some significant differences in the mass spectra of the two isomers were detected and it has been concluded that the isomerisation of i-butane to n-butane is a process which is faster than the duration of the laser pulse used.

Production of the NO photofragment in the desorption of RDX and HMX from surfaces

A promising scheme for the remote detection of nitrate-based explosives, which have low vapor pressure, involves two lasers: the first to desorb, vaporize, and photofragment the explosive molecule and the second to create laser-induced fluorescence in the NO fragment. It is desirable to use for the first a powerful 532 nm frequency-doubled Nd:YAG laser. In this study, we investigate the degree of photofragmentation into NO resulting from the irradiation of the explosives RDX and HMX coated on a variety of surfaces. The desorption step is followed by femtosecond laser ionization and time-of-flight mass spectrometry to reveal the fragments produced in the first step. We find that modest laser power of 532 nm desorbs the explosive and produces adequate amounts of NO.

Application of femtosecond laser mass spectrometry to the analysis of volatile organic compounds

Femtosecond (fs) lasers have high intensity and ultrashort pulse duration. Tunneling ionization occurs for molecules subject to such intense laser fields. We have studied the mass spectra of a variety of molecules irradiated by intense fs laser pulses. These molecules include some typical volatile organic compounds contained in human breath and in the atmosphere. The results demonstrate that all of these molecules can be ionized by intense fs laser pulses. Dominant parent ion and some characteristic ionic fragments are observed for each molecule. The degree of fragmentation can be controlled by adjusting the laser intensity. Moreover, saturation ionization can occur for each molecule by increasing the laser intensity. These features indicate that fs laser mass spectrometry can be a sensitive tool to identify and quantify volatile organic compounds in human breath.

Electron induced dissociation in the condensed-phase nitromethane: II. Desorption of neutral fragments

Low energy electron induced dissociation in multilayer films of nitromethane (CD3NO2) was investigated by high resolution electron energy loss spectroscopy (HREELS) and by the electron stimulated desorption (ESD) of neutral species. HREELS measurements show that the lowest electronic states of the condensed molecule are very similar to those seen in the gas phase. Desorbed neutrals were detected using combined non-resonant multi-photon ionization at 355 nm and time of flight mass spectrometry. The most intense signals detected were those of CD3 (+) and NO (+) and are attributed primarily to the desorption of CD3 and NO2 fragments following molecular dissociation via low-lying electronic excited states of nitromethane (the detected NO (+) being the result of the dissociative ionization of NO2). By varying the time delay between the incident electron pulse and the ionizing laser pulse, it is possible to measure the kinetic energy distributions of desorbing fragments. The kinetic energy distributions above ∼ 5 eV appear invariant with incident electron energy, indicating that the same desorption process (dissociation via low-lying electronic states) operates at all the studied incident energies. Nevertheless, measurements of neutral yields as functions of incident electron energy demonstrate that excitation of the dissociative electronic states also proceeds via previously identified transient negative ions. At energies less than ∼ 5 eV, contributions from dissociative electron attachment are also observed in the yield of CD3 and other neutral fragments.

Theoretical prediction regarding structural and thermodynamical characteristics of stable CH3PO2 isomers and unimolecular decomposition mechanisms of species CH3P(=O)2, CH3O-P=O, and CH2=P(=O)OH

The detailed isomerization and dissociation reaction potential energy profile of the CH(3)PO(2) system was established at the UCCSD(T)/6-311++G(3df,2p)//UB3LYP/6-311++G(d,p) level of theory. Seventy minimum isomers were located and connected by 93 optimized interconversion transition states. Furthermore, 32 isomers with high kinetic stability were predicted to be possible candidates for further experimental detection. The bonding nature of the suggested stable isomers was analyzed while their molecular properties including heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities were calculated at the G2, G2(MP2), G3, and CBS-Q levels. Based on the isomerization and dissociation potential energy surface, possible unimolecular decomposition mechanisms and pathways of the low-lying molecules CH(3)P(=O)(2), CH(3)O-P=O, and CH(2)=P(=O)OH were discussed. Furthermore, the transition state theory rate constants of the primary unimolecular dissociation channels were also calculated.

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

The ultrafast laser-induced photoionization and photodissociation processes of the nitroaromatic containing explosive and explosive related compounds (ERCs) nitrobenzene (NB), 1,3-dinitrobenzene (DNB), m-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) have been investigated at three laser wavelengths and power densities using a time-of-flight mass spectrometer. Examination of the mass spectra of these compounds reveals the enhanced formation of the molecular ion [M(+)] when ultraviolet (332 nm) and visible (495 nm) light is used relative to infrared (795 nm) radiation. In addition, at 795 nm and a power density of 3.5 x 10(14) W/cm(2), the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated.

Effect of Cation Absorption on Ionization/Dissociation of Cycloketone Molecules in a Femtosecond Laser Field

The mass spectra of a series of cycloketone molecules, cyclopentanone (CPO), cyclohexanone (CHO), cycloheptanone (CHPO), and cyclooctanone (COO) are measured in a 788 or 394 nm laser field with 90 fs pulse duration and the intensity ranging from 5 x 10(13) W/cm(2) to 2 x 10(14) W/cm(2). At 788 nm, a dominated parent ion peak and some weak peaks from the fragment ions C(n)H(m)+ are observed for CPO and CHO (a ratio P(+)/T(+), the parent ion yield to the total ion yield, is 81.6% and 52.6%, respectively). But the extensive fragment ion peaks are observed with the greatly reduced parent ion peak for CHPO (P(+)/T(+) = 5.5%) and that are even hard to be identified for COO. These observations are interpreted explicitly in the frame of the significant resonant effect of their cation photoabsorption on ionization and dissociation of these molecules. The present work also suggests that a nonadiabatic ionization occurs with a nuclear rearrangement due to the H movement in these molecules during the ionization in an intense femtosecond laser field.

Role of surface crossings in the photochemistry of nitromethane

The photodissociation dynamics of nitromethane (CH(3)NO(2)) starting at the S(3) excited state has been studied at the complete active space self-consistent field level of theory in conjunction with atomic natural orbital type basis sets. In addition, the energies of all the critical points and the energy profiles connecting them have been recomputed with the multiconfigurational second-order perturbation method. It is found that the key step in the reaction mechanism is a radiationless decay through an S(3)S(2) conical intersection. The branching space spanned by the gradient difference and nonadiabatic coupling vectors of this crossing point comprises dissociation into excited nitromethane plus singlet atomic oxygen [CH(3)NO(1A")+O((1)D)] and S(3)-->S(2) deactivation, respectively. Furthermore, deactivated nitromethane S(n (n<3)) can decompose in subsequent steps into CH(3)+NO(2), where NO(2) is generated at least in two different electronic states (1 (2)B(2) and 1 (2)A(1)). It is shown that formation of excited nitric oxide NO(A (2)Sigma) arises from CH(3)NO(1A") generated in the previous step. In addition, four crossings between singlet and triplet states are localized; however, no evidence is found for a relevant role of such crossings in the photochemistry of CH(3)NO(2) initiated at S(3) state in the gas phase.

Interaction Mechanism of Some Alkyl Iodides with Femtosecond Laser Pulses

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 x 10(13-)5 x 10(15) W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values gamma approximately 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail.

Explosives detection: a challenge for physical chemistry

The detection of explosives, energetic materials, and their associated compounds for security screening, demining, detection of unexploded ordnance, and pollution monitoring is an active area of research. A wide variety of detection methods and an even wider range of physical chemistry issues are involved in this very challenging area. This review focuses on techniques such as optical and mass spectrometry and chromatography for detection of trace amounts of explosives with short response times. We also review techniques for detecting the decomposition fragments of these materials. Molecular data for explosive compounds are reviewed where available.

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 x 10(9) W cm(-2) with ionisation laser intensities between 2 x 10(14) and 6 x 10(15) W cm(-2). Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.