Merrick J. DeWitt

Near‐infrared femtosecond photoionization/dissociation of cyclic aromatic hydrocarbons

Pulses of 780 nm light of duration 170 fs and power densities up to 3.8×1013 W cm−2 are used to study the photoionization/dissociation processes in the series of gas phase, cyclic aromatic hydrocarbons including benzene, naphthalene, phenanthrene, and anthracene. The near‐infrared ionization process leads to the production of intact molecular ions for all of the molecules studied. Measurements of the ion intensity as a function of laser fluence revealed the order of the ultrafast ionization process to be 8.0±0.1 for anthracene, 6.9±0.1 for phenanthrene, 8.5±0.1 for naphthalene, and 8.1±0.1 for benzene. The relative femtosecond photoionization cross section decreased from 1.0 for anthracene to 0.2 for phenanthrene to 0.1 for naphthalene to ∼0.005 for benzene. The relative order and cross section of the femtosecond ionization processes suggest that a field ionization mechanism is operative.

Photoionization/dissociation of alkyl substituted benzene molecules using intense near-infrared radiation

Abstract The photoionization products for benzene, toluene, ethylbenzene and n-propylbenzene are measured using time-of-flight methods upon interaction with 780 nm radiation pulses of duration 170 fs and intensity 10 13-13.6 W cm −2 . The relative ionization yields scale as 1, 1.2, 1.4, and 0.35 for benzene, toluene, ethylbenzene and n-propylbenzene at maximum laser intensity. Limited dissociation was observed for benzene and toluene at all laser power densities. A significant (>5%) dissociation/ionization yield was observed for ethylbenzene and n-propylbenzene at the lowest laser power densities. The dissociation increased as 11th order and 9th order processes for ethylbenzene and n-propylbenzene, respectively to a maximum of 70% dissociation/ionization for n-propylbenzene at 3.8 × 10 13 W cm −2 . At 3.8 × 10 13 W cm −2 the dissociation yield increases quadratically with alkyl chain length for the series. The observed ionization/dissociation trends are consistent with a model which incorporates both field ionization and energy redistribution concepts.

Concerning the ionization of large polyatomic molecules with intense ultrafast lasers

The relative photoionization/dissociation probabilities are presented for the molecules benzene, naphthalene, and anthracene upon interaction with 780 nm laser radiation of duration 170 fs and intensity 3.8×1013 W cm−2. Both the ionization probability and the dissociation yield increase exponentially from benzene to anthracene as measured by time-of-flight mass spectra. A structure-based model is presented for the excitation of large polyatomic molecules by intense laser irradiation with pulse widths on the time scale of molecular vibration (100 fs) and with peak field strengths of 1–2 V A−1. The model accounts for molecular structure and is able to accurately predict the transition from multiphoton ionization (MPI) to tunnel ionization. It is also demonstrated that this structure-based model can quantitatively predict the experimentally measured ionization probabilities. In comparison, models employing the more conventional zero-range potential do not accurately predict either the transition or the relat...

Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules

A numerical model is presented to determine the Keldysh adiabaticity parameter for the interaction of an intense laser with a polyatomic molecule. The adiabaticity parameter is a guide to determining whether the ionization process is in the field or multiphoton ionization regime. The adiabaticity parameters are compared for potentials including the simple zero-range potential, the Coulomb potential, an atomic potential (Xe), a diatomic (N2) molecular potential, and a polyatomic (C6H6) molecular potential. It is demonstrated that the Coulomb potential is approximately equal to the atomic and diatomic potentials and differs from the zero-range potential employed in the Keldysh model in a way which is predominantly dependent upon the ionization potential. Both simple models substantially overestimate the adiabaticity parameter for C6H6 at all field strengths and at fields above 1.25 V/A both simple models become completely unphysical. This is because barrier suppression ionization is predicted to occur for b...

THE ROLE OF ELECTRON DELOCALIZATION IN THE IONIZATION OF C6 HYDROCARBONS USING INTENSE 780 NM LASER PULSES OF FEMTOSECOND DURATION

The photoionization/dissociation mass spectra are reported for the series of molecules benzene (C6H6), 1,3,5-hexatriene (C6H8), cyclohexane (C6H12), and n-hexane (C6H14) as a function of laser power intensity from 1 to 3.8×1013 W cm−2 using a pulse duration of 170 fs and wavelength 780 nm. The ionization orders are localized around 8.3 for benzene, 1,3,5-hexatriene, and cyclohexane and the relative ionization probabilities are measured to be 1,79, and 0.15, respectively. No ion current was observed for n-hexane. The dissociation yield is observed to increase exponentially as a function of the number of atoms in the molecule with cyclohexane undergoing the most dissociation and benzene undergoing essentially no dissociation. These observations are interpreted in light of a field ionization model that incorporates both the ionization potential and the electronic and nuclear structure of the molecule.

Predicting intense field laser ionization probabilities: The application to C2Hn species

A structure-based tunneling mechanism developed to predict the ionization of molecules subjected to intense, ultrafast irradiation is tested on the series of C2 hydrocarbons: acetylene, ethylene, and ethane. Relative ionization probabilities (1, 4.1, and 8.7 for ethane, acetylene, and ethylene, respectively) are measured upon excitation with 780 nm, 125 fs pulses of 6×1013 W cm−2 and compared to predictions of the model (1, 4.1, and 7.9 for ethane, acetylene, and ethylene, respectively). Ionization probabilities determined via the ADK (Ammosov, Delone, and Krainov) model for atomic ionization (1, 2.7, and 13.1 for ethane, acetylene, and ethylene, respectively) are shown to be near those of the structure-based model.

Orientational averaging in the intense field tunnel ionization of molecules

The effect of molecular structure and orientation on the rate of tunnel ionization in intense laser fields is considered for linear molecules. The nonspherical nature of the molecular electrostatic potential near the classical electron turning point results in a nonisotropic distribution of tunneling rates. This must be considered when determining the overall ionization rate of a randomly oriented ensemble. Calculations are performed for two atom:molecule test pairs with similar ionization potential, Ar:N2 and Xe:O2, and the results are compared to experiments [Talebpour, Chien, and Chin, J. Phys. B 29, L677 (1996); Guo et al., Phys. Rev. A 58, R4271 (1998)].

Photoionization of Polyatomic Molecules Using Intense, Near‐Infrared Radiation of Femtosecond Duration

The relative crossections for the photoionization and the photodissociation of a series of aromatic compounds is presented in this paper. The molecules are ionized using an intense pulse of 780 nm radiation of duration 170 fs. The photo‐induced processes are measured using time‐of‐flight mass spectrometry. We find that the photoionization rate scales as e‐ionization potential. We propose that the reason for the exponential dependence has to do with the three dimensional structure of the molecule under investigation.

High-speed DNA sequencing in the gas phase

Efficient sequencing of the human genome will require the development of new methods that are less expensive and orders of magnitude faster than current technology. Recent advances in laser-based methodology suggest that a mass spectroscopic DNA sequencing technique may surmount present limitations. This contribution will focus on the use of laser vaporization and laser ionization to prepare single stranded DNA for high speed sequencing in the gas phase. As a first step in the implementation of a mass spectroscopic sequencing approach, we have shown that single-stranded DNA molecules having chain lengths of over 1000 nucleotides can be laser vaporized into the gas phase with no discernible strand cleavage. This observation provides the basis for the time-of-flight (TOF) mass spectral-based sequencing experiment that we are developing. To determine the DNA sequence the experiment will be repeated for the four complimentary dideoxy sequencing reactions. The realization of this method would allow a 300 base DNA sequence to be determined in less than one second. At that rate, an instrument based on this technology could potentially generate sequencing data in excess of 25 million bases per day.