Multiple Photon Ionization Research Articles

Real time monitoring of slow pyrolysis of polyethylene terephthalate (PET) by different mass spectrometric techniques

In the context of waste upgrading of polyethylene terephthalate (PET) by pyrolysis, this study presents three on-line mass spectrometric techniques with soft ionization for monitoring the emitted decomposition products and their thermal dependent evolution profiles. Pyrolysis experiments were performed using a thermogravimetric analyzer (TGA) under nitrogen atmosphere with a heating rate of 5 °C/min from 30 °C to 600 °C. Single-photon ionization (SPI at 118 nm/10.5 eV) and resonance enhanced multiple photon ionization (REMPI at 266 nm) were used with time-of-flight mass spectrometry (TOF-MS) for evolved gas analysis (TGA-SPI/REMPI-TOFMS). Additionally, the chemical signature of the pyrolysis products was investigated by atmospheric pressure chemical ionization (APCI) ultra high resolution Fourier Transform ion cyclotron resonance mass spectrometry (FT-ICR MS) which enables assignment of molecular sum formulas (TGA-APCI FT-ICR MS). Despite the soft ionization by SPI, the fragmentation of some compounds with the loss of the [O-CH = CH2] fragment is observed. The major compounds were acetaldehyde (m/z 44), benzoic acid (m/z 122) and a fragment of m/z 149. Using REMPI, aromatic species were selectively detected. Several series of pyrolysis products were observed in different temperature intervals, showing the presence of polycyclic aromatic hydrocarbons (PAHs), especially at high temperatures. FT-ICR MS data showed, that the CHO4 class was the most abundant compound class with a relative abundance of 45.5%. The major compounds detected with this technique corresponded to m/z 193.0495 (C10H9O4+) and 149.0233 (C8H5O3+). Based on detailed chemical information, bulk reaction pathways are proposed, showing the formation of both cyclic monomer/dimer and linear structures.

Influence of pulse duration on above-threshold ionization in intensive circularly polarized laser field

The above-threshold ionization (ATI) process of argon in intensive circularly polarized laser field has been investigated by solving the time-dependent Schrödinger equation in length gauge with emphasis on pulse duration effect. It is shown that the influence of pulse duration is obvious on the transition between multiple-photon and tunneling ionization regimes when the Keldysh parameter is no more than 3.25. That is, photoelectron ionization by shorter pulse occurs in the tunneling regime mainly, and yet the multiple-photon absorption appears more easily with longer pulse. However, such influence disappears when the Keldysh parameter is sufficiently large and the ATI always appears in the multiple-photon ionization regime.

Communication: IR spectroscopy of neutral transition metal clusters through thermionic emission

The resonant multiple photon excitation of neutral niobium clusters using tunable infrared (IR) radiation leads to thermionic emission. By measuring the mass-resolved ionization yield as a function of IR wavenumber species selective IR spectra are obtained for Nb(n) (n = 5-20) over the 200-350 cm(-1) spectral range. The IR resonance-enhanced multiple photon ionization spectra obtained this way are in good agreement with those measured using IR photodissociation of neutral Nb(n)-Ar clusters. An investigation of the factors determining the applicability of this technique identifies the internal energy threshold towards thermionic emission in combination with a minimum required photon flux that rapidly grows as a function of excitation wavelength.

Intensity-resolved IR multiple photon ionization and fragmentation of C60

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

UV multiphoton fragmentation and ionization of SF5 radicals and highly vibrationally excited SF6 molecules

Abstract The mechanism whereby the S + and SF + ions are formed as a result of the combined IR + UV irradiation of jet-cooled SF 6 molecules is studied both experimentally and theoretically. It is demonstrated that there exist at least two channels for these ions to form. The first is the UV multiphoton fragmentation and ionization of the SF 5 radicals formed in the course of the IR multiple-photon (MP) dissociation of the SF 6 molecules. The second starts with the UV excitation of the highly vibrationally excited SF 6 molecules formed during IR MP excitation. A theoretical model of the process is developed, describing in particular the dynamics of the IR MP excitation and dissociation of the SF 6 molecules and the subsequent space–time evolution of the products. This model is used to find the efficiency of the UV MP ionization of SF 6 molecules as a function of their vibrational energy. At λ =232 nm this process starts with the SF 6 molecules having a vibrational energy of E vib ≳ (15–20)×10 3 cm −1 .

Single and multiple photon ionization of triethylamine

Single and multiple photon ionization photoelectron spectroscopy of triethylamine (TEA) was studied using a newly developed high-resolution electron spectrometer which utilizes position sensitive detection. The adiabatic ionization potential of TEA was accurately determined using both single (7.47±0.04 eV) and multiphoton (7.53±0.10 eV) ionization photoelectron spectroscopy. Although excitation to both the S1 and S2 states can occur, multiphoton ionization always occurs out of the S1 state. When the cation dissociates, the distribution of photoelectron energies similarly reflects this partitioning between S1 and S2.

Femtosecond Time-Delayed Photoionization Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene

This paper presents results from femtosecond time-resolved studies of the ultrafast internal conversion in 1,3,5-hexatriene following excitation of the 1{sup 1}B{sub u} state at around 250 nm. The internal conversion process is investigated using time-delayed single and multiple photon ionization with 350 nm and 310 nm probe pulses. The initial step in the internal conversion is found to be very fast (<100 fs) as expected from absorption line width measurements. However we are able to observe the subsequent time evolution of the molecule for more than 1 ps. The internal conversion appears to proceed through an intermediate state, likely to be the 2{sup 1}A{sub g} electronic state, that is seen to decay with a approximately 250 fs time constant. 44 refs., 8 figs.

Surface analysis of bulk polymers using single- and multiple-photon ionization: U Schühle et al, J Vac Sci Technol, A6, 1988, 936–940

Surface analysis of bulk polymers using single- and multiple-photon ionization: U Schühle et al, J Vac Sci Technol, A6, 1988, 936–940

Surface analysis of bulk polymers using single- and multiple-photon ionization

The surface analysis by laser ionization (SALI) technique has been applied to the chemical analysis of bulk solid polymer surfaces. Surface material was removed by a pulsed rare-gas ion probe beam at ∼7 keV and sputtered neutral molecules were photoionized by pulsed coherent radiation at 118 nm (10.5 eV) or 248 nm (5.0 eV). Photoions were then extracted into a reflecting time-of-flight mass spectrometer. The ion beam dose during analysis was kept below significant damage levels (&amp;lt;1013 primary ions/cm2), indicative of the very high sensitivity. The single-photon ionization (118 nm) resulted in much more extensive and representative mass spectra than for the multiphoton ionization, reflecting the fact that it is a relatively ‘‘soft’’ ionization technique. Very distinct and readily interpretable mass spectra are shown for poly(tetrafluoroethylene), the polyamide nylon 66, and poly(ethylene-glycol)-monostearate. Where comparison secondary ion mass spectra are available, these SALI spectra with one-photon ionization compare quite favorably.

ChemInform Abstract: Two‐Photon Resonance REMPI Detection of the Formyl Radical.

The resonance enhanced multiple photon ionization (REMPI) spectrum of HCO has been observed for dye laser wavelengths from 373 to 427 nm. Long progressions in the bending vibration have been observed for excitation of two electronic states, tentatively identified as the 3p 2Σ+ and 3p 2Π Rydberg states. Estimates are obtained for the rotational constants, vibrational frequencies, anharmonicity constants, and electronic term values for the two states. Additional strong REMPI signals were obtained in the 418–426 nm region, tentatively assigned to the 3s 2Σ+ Rydberg state. A single pulse detection limit for HCO of 1013 cm−3 was measured which suggests the feasibility of the REMPI detection of HCO at the part‐per‐million level in atmospheric pressure hydrocarbon flames.

Two-photon resonance REMPI detection of the formyl radical

The resonance enhanced multiple photon ionization (REMPI) spectrum of HCO has been observed for dye laser wavelengths from 373 to 427 nm. Long progressions in the bending vibration have been observed for excitation of two electronic states, tentatively identified as the 3p 2Σ+ and 3p 2Π Rydberg states. Estimates are obtained for the rotational constants, vibrational frequencies, anharmonicity constants, and electronic term values for the two states. Additional strong REMPI signals were obtained in the 418–426 nm region, tentatively assigned to the 3s 2Σ+ Rydberg state. A single pulse detection limit for HCO of 1013 cm−3 was measured which suggests the feasibility of the REMPI detection of HCO at the part-per-million level in atmospheric pressure hydrocarbon flames.

Laser Photoionization Spectroscopy of Single Atoms and Molecules

A new, highly sensitive method of laser spectroscopy of atoms and molecules based on multiple photon ionization is described. The history, characteristics and some key applications of this method are presented.

Collision-free multiple photon ionization of atoms and molecules at 193 nm.

The nonlinear coupling of 193-nm radiation to a range of atomic and molecular materials has been experimentally explored up to a maximum intensity on the order of \ensuremath{\sim}${10}^{17}$ W/${\mathrm{cm}}^{2}$. Studies of collision-free ion production clearly exhibit anomalous behavior which strongly implies that the atomic shell structure is the principal determinant in the observed response. On the basis of the observed coupling strength and the measured atomic-number (Z) dependence, the experimental evidence points to a coherent atomic motion involving several electrons, possibly an entire shell, as the main physical mechanism enabling the scale of energy transfers seen. Therefore, states representing multiple excitations appear to play a central role in the coupling, a consideration that fundamentally distinguishes the nonlinear interaction of a multielectron atom from that of a single-electron system. Comparison of the experimental findings with standard theoretical treatments, of either a perturbative or nonperturbative nature, does not produce satisfactory agreement. Conversely, the formulation of a simple classical estimate qualitatively conforms to several features of the observed behavior including the shell character of the interaction, the maximum energy transfer, the dependence of the average energy transfer on the intensity of irradiation, the frequency dependence of the observed energy transfer, and the weak influence of polarization.

Relativistic Derivation of the Ponderomotive Force Produced by Two Intense Laser Fields

The ponderomotive force plays a fundamental role in the absorption of laser light on self-consistent plasma density profiles, in multiple-photon ionization, and in intense field electrodynamics. The relativistic corrections to the ponderomotive force of a transversely polarized electromagnetic wave lead to an approximately 20-percent reduction in the single particle ponderomotive force produced by a 10-μm 1016-W/cm2 laser field. Recent experimental investigations are based on using two intense laser fields to produce desired laser-matter interactions. This paper presents the first derivation of the nonlinear relativistic ponderomotive force produced by two intense laser fields. The results demonstrate that relativistic ponderomotive forces are not additive.

Species density measurements with the rempi method; The detection of CO and C 2O in a methane/oxygen flame

Within the past two years resonance enhanced multiple photon ionization (REMPI) methods have been used for the sensitive detection of several radicals and molecules of importance in combustion chemistry. The use of REMPI methods is illustrated with a discussion of recent density measurements of NO molecules and H atoms. We report here the first REMPI detection of CO and C 2 O in a low pressure CH 4 /O 2 flame. Other species previously detected in flame environments include H, O, CH, NO, and PO.

Relativistic corrections to the electron energy spectrum of multiphoton ionization

The energy spectrum of electrons produced by multiple photon ionization is modified by the acceleration of the free, final-state electrons in the field associated with the intense, ionizing laser. This acceleration is due to the field-gradient force and the relativistic corrections to this force are derived herein. It is demonstrated that they are substantial for presently available lasers.

Atomic ionization by intense laser fields

A method, alternate to that of conventional perturbation theory, is developed and illustrated for the computation of the photoionization of atoms by photons with energies less than the atomic ionization potential. The method allows treatment of the electromagnetic field to all orders at a rather simple level of analysis. It represents, in fact, a slight generalization of the Born approach to the theory of scattering. As a consequence of certain approximations, simple analytic formulae are obtained to describe multiple photon ionization events. Because of one of the approximations (a situation straightforwardly rectifiable, but not attempted here for the sake of illustrative simplicity), the formulae obtained here are probably only qualitatively interesting.