| 1. |
- Boyle, Mark, et al.
(författare)
-
Excitation dynamics of Rydberg states in C60
- 2005
-
Ingår i: European Physical Journal D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 36, s. 339-351
-
Tidskriftsartikel (refereegranskat)abstract
- The electron and nuclear dynamics of C60 fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C60. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (hu) → LUMO+1 (t1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C60 to short-pulse laser radiation.
|
|
| 2. |
- Boyle, Mark, et al.
(författare)
-
Two colour pump-probe study and internal energy dependence of Rydberg state excitation
- 2004
-
Ingår i: Physical Review A. - 1050-2947. ; 70
-
Tidskriftsartikel (refereegranskat)abstract
- Excitation of Rydberg states in isolated C60 is studied by time-resolved photoelectron spectroscopy in a femtosecond two-color pump-probe experiment. The relaxation time for electron-electron interaction is determined to be approximately 100 fs with the t1g(LUMO + 1) orbital being considered to define the doorway state in a nonadiabatic multielectron excitation process. The internal energy stored in vibrational modes of the C60 at 770 K is found to support the excitation process very efficiently while in "cold" C60 (80 K), no significant Rydberg population is detected.
|
|
| 3. |
- Bulgakova, Nadya, 1956, et al.
(författare)
-
A general continuum approach to describe fast electronic transport in pulsed laser irradiated materials: the problem of Coulomb explosion
- 2005
-
Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 81, s. 345-356
-
Tidskriftsartikel (refereegranskat)abstract
- We present a continuum model, based on a drift-diffusion approach, aimed at describing the dynamics of electronic excitation, heating, and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, for near-infrared femtosecond irradiation, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, application of the model to UV nanosecond pulsed laser interaction with bulk silicon has pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) and material properties are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics.
|
|
| 4. |
- Bulgakova, Nadya, 1956, et al.
(författare)
-
Electronic transport and consequences for material removal in ultrafst pulsed laser ablation of materials
- 2004
-
Ingår i: Physical Review B. - 1098-0121. ; 69
-
Tidskriftsartikel (refereegranskat)abstract
- Fast electronic transport is investigated theoretically based on a drift-diffusion approach for different classes of materials (metals, semiconductors, and dielectrics) under ultrafast, pulsed laser irradiation. The simulations are performed at intensities above the material removal threshold, characteristic for the ablation regime. The laser-induced charging of dielectric surfaces causes a subpicosecond electrostatic rupture of the superficial layers, an effect which, in comparison, is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties.
|
|
| 5. |
- Bulgakova, Nadya, 1956, et al.
(författare)
-
Model description of surface charging during ultrafast pulsed laser ablationof materials
- 2004
-
Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 79, s. 1153-1155
-
Tidskriftsartikel (refereegranskat)abstract
- We present a model describing the dynamical mechanisms responsible for generating fast ion ejection under ultra-short pulsed laser irradiation. The model is based on a simplified drift–diffusion approach describing the evolution of the laser-generated charge carriers, their transport, and the electric field generated as a result of quasi-neutrality breaking in the irradiated target. The importance of different processes in generating the non-thermal material-ejection mechanisms is discussed. A common frame is applied to dielectrics, semiconductors, and metals and different dynamical behaviour is observed. The modelling results are in good agreement with fs pump–probe studies and measurements of the velocity distributions of the emitted ions.
|
|
| 6. |
- Bulgakova, Nadya, 1956, et al.
(författare)
-
Surface charging under pulsed laser ablation of solids and its consequences: studies with a continuum approach
- 2005
-
Ingår i: SPIE Proceedings. - : SPIE. - 0277-786X. ; 5714, s. 9-23
-
Tidskriftsartikel (refereegranskat)abstract
- Dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond pulsed laser irradiation is studied based on a unified continuum model. A simplified drift-diffusion approach is used to model the energy flow into the sample in the first hundreds of femtoseconds of the interaction. The laser-induced charging of the targets is investigated at laser intensities slightly above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. Various related aspects concerning the possibility of CE for different irradiation parameters (fluence, wavelength and pulse duration) as well as the limitations of the model are discussed. These include the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the optical (reflection and absorption) characteristics. A controversial topic concerning CE probability in laser irradiated semiconductor targets is also a subject of this work.
|
|
| 7. |
- Hansen, Klavs, 1958, et al.
(författare)
-
Laser power dependence in femtosecond ionization of fullerenes
- 2005
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 42, s. 282-285
-
Tidskriftsartikel (refereegranskat)abstract
- Fullerene ionization can under some circumstances be modelled as thermal emission of electrons from a transiently hot electron gas. Application of this idea to experiments involving femtosecond lasers gave theoretical evidence for ion yields that would vary with the pulse energy of the laser to some power, identical to the behavior seen in multi-photon experiments. The reason for this behavior is investigated here. The crucial component is identified as the Poisson statistics for photon absorption and a strong variation of the ion yield with energy.
|
|
| 8. |
- Lassesson, Andreas, 1976, et al.
(författare)
-
A femtosecond laser study of the endoral fullerenes Li@C60 and La@C82
- 2005
-
Ingår i: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 34, s. 205-209
-
Tidskriftsartikel (refereegranskat)abstract
- The endohedral fullerenes La@C82 and Li@C60 have been studied in the gas phase with femtosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C82 fragmenting via the shrink-wrap mechanism and Li@C60 predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C82 agree well with a recently developed model for fs laser ionisation of C60.
|
|
| 9. |
- Stoian, Razvan, et al.
(författare)
-
Comment on Coulomb explosion in femtosecond laser ablation of Si(111)
- 2004
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85, s. 694-695
-
Tidskriftsartikel (refereegranskat)abstract
- In a recent letter Roeterdink 1 report on the occurrence of an electrostatic form of material removal from solid silicon samples irradiated with high-intensity ultrashort laser pulses. The arguments are essentially derived from time-of-flight observations of the emitted single- and double-ionized silicon atoms and considerations related to momentum conservation during the excitation and expansion phase. A linear scaling of the velocity of species with different charge states is considered as an argument for Coulomb explosion being responsible for ion emission from the irradiated silicon sample in a high fluence regime. A similar effect derived directly from momentum conservation upon particle ejection has been determined before as a proof for the occurrence of Coulomb explosion from laser irradiated dielectrics,2,3 but a similar process for semiconductors was not observed at intensities just above the ablation threshold.3 The absence of Coulomb explosion from semiconductors within the range of investigated fluences (up to 1 J/cm2) was related to efficient electronic transport able to counterbalance the laser-induced charge deficiency. The "discrepancy" discussed in the letter, involving a threshold criterion for the surface Coulombic explosion, is based on a misinterpretation of the data presented by Stoian 3 Usually the mechanism of Coulomb explosion is explained as being driven by positively charged superficial layers that will electrostatically repel each other, assisted in some cases by the pulling force exercised by the photo-emitted charge cloud close to the ionized surface. Recent calculations4 regarding the mechanisms for surface electrostatic disruption have shown that Coulomb explosion may occur even without the additional electron pull. Provided that substantial photoelectron emission occurs and carrier mobility is intrinsically low or lowered during the excitation, the electron transport from the excited bulk region cannot compensate the photoelectric flow, and, therefore, the surface neutrality will be broken. A significant uncompensated remnant positive charge will be localized within the first surface layers and the ions within this region will be mutually repelled. The authors have overlooked, when referring to the results of Stoian ,3 the fact that in subsurface regions where photoemission is less effective one can also obtain a high density of carriers. The discussion and arguments in Ref. 3 refer to the net charge (the absolute difference between the ion and electron density) in the surface layers, with no a priori restrictions for the absolute carrier density to reach supercritical values at the irradiation wavelength. According to the study of Roeterdink , the high carrier density induced by the ultrafast laser excitation, in excess of 1022 cm–3 will destabilize the lattice, leading to the surface electrostatic disruption. A fractional charge of more than 0.3 excited electrons per silicon atom is calculated to generate electric fields matching the observed momentum transfer, but the local neutrality of the sample appears not to be disturbed. Lattice destabilization at high carrier density is well documented in the literature5 but it does not involve any neutrality breakdown that may cause electrostatic material ejection. It is thus unclear how high carrier densities alone, even close to the solid density, may lead to high electrostatic fields and Coulomb explosion of the region, without any deviation from the electric neutrality. One may invoke either strong photoelectron emission or charge separation caused by nonequilibrium transport, but this is not clearly discussed in the letter. If the authors imply that high excitation simultaneously means high photoelectron yields, a net, uncompensated charge of more than 0.3 missing electrons per atom can be reached in the present case only if one assumes that all the excited electrons have been removed from the superficial layers, without any other forms of electronic supply during the emission time. If one considers that the excitation depth in silicon is several hundreds of nanometers and the electrons can be removed with a certain probability only from a narrow region beneath the surface, the uncompensated region will be rapidly neutralized by bulk electronic transport, keeping the net charge below the critical value for Coulomb explosion. Though the possibility to efficiently charge the Si surface layers beyond the threshold for bond-breaking and macroscopic rupture of the surface may conceivably appear under extreme irradiation conditions involving high intensities where high carrier densities are generated on the leading edge of the pulse and electronic transport is strongly perturbed in the destablized lattice, the experimental results are usually embedded in a series of additional effects and artifacts in the laser-generated plume that may obscure a clear interpretation of the ejection mechanisms. Moreover, one has to carefully consider the changes that occur in the irradiation geometry once high fluences and high irradiation doses are used. Here, one of the consequences is indicated by the authors in the appearance of a "second ejection channel." We suggest a possible alternative explanation for the experimental facts observed by the authors. Under the experimental conditions reported, i.e., fluence in the range of 3–10 J/cm2, several times higher than the ablation threshold, the material removal rates are considerable. An immediate effect, accentuated by the production of a crater at the surface (as acknowledged by the authors) after several laser pulses, is a highly collisional, dense plume, initially confined in the crater before expanding. Consequently charge separation may occur in the gas phase,6 leading to a similar experimental signature, namely a linear scale of the velocity of different ionized species with their intrinsic charge. More explicitly, the appearance of the so-called double layer will accelerate multiple charges and the same momentum regulations occur.6
|
|
