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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 11 — May. 24, 2010
  • pp: 11664–11670
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Molecular high harmonic generation in a two-color field

Pengfei Wei, Candong Liu, Chunmei Zhang, Yansui Huang, Yuxin Leng, Peng Liu, Yinghui Zheng, Zhinan Zeng, Ruxin Li, and Zhizhan Xu  »View Author Affiliations


Optics Express, Vol. 18, Issue 11, pp. 11664-11670 (2010)
http://dx.doi.org/10.1364/OE.18.011664


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Abstract

We experimentally investigate the high harmonic generation (HHG) from CH4 molecules and Xe atoms in a two-color field (using the 800nm laser and the tunable laser with the longer wavelength from 1500nm to 1900nm), and observe that the longer wavelength component can destructively suppress the HHG from CH4 molecules. By controlling the time delay between the two color laser pulses or tuning the laser intensity of the longer wavelength component, the suppressions of the HHG from CH4 molecules and the enhancements of the HHG from Xe atoms at the same laser condition are observed. The results indicate that the longer wavelength component around the molecular infrared absorption can suppress the molecular HHG process.

© 2010 OSA

1. Introduction

High harmonic generation (HHG) in atoms and molecules has been explored broadly in the past decade. Many new physical phenomena in the high-field interaction are discovered, and a robust ultrafast coherent radiation source in the XUV region has been developed [1

1. P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett. 71(13), 1994–1997 (1993). [CrossRef] [PubMed]

,2

2. G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, “Isolated single-cycle attosecond pulses,” Science 314(5798), 443–446 (2006). [CrossRef] [PubMed]

]. In the semiclassical three-step model of HHG, active electrons first tunnel through the potential barrier, are then accelerated by laser field, and finally recombine with parent ions to emit high-energy photons. This process can provide an approach to probe the electronic and nuclear dynamics of atoms and molecules with the unprecedentedly temporal resolution, and also offer an insight into the transient molecular structure [3

3. T. Kanai, S. Minemoto, and H. Sakai, “Quantum interference during high-order harmonic generation from aligned molecules,” Nature 435(7041), 470–474 (2005). [CrossRef] [PubMed]

7

7. S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirila, M. Lein, J. W. G. Tisch, and J. P. Marangos, “Probing proton dynamics in molecules on an attosecond time scale,” Science 312(5772), 424–427 (2006). [CrossRef] [PubMed]

] since the harmonic emission depends on the molecular structure.

In this work, we experimentally investigate the HHG from CH4 molecules and Xe atoms in a two-color field by controlling the time delay between the two color laser pulses or tuning the laser intensity of the longer wavelength component. The suppression of the HHG from CH4 molecules is researched, and the enhancement of the HHG from Xe atoms at the same laser condition is also observed as comparison. These results indicate that the longer wavelength component around the molecular infrared absorption can destructively suppress the HHG from CH4 molecules. Finally, we experimentally demonstrate the wavelength effect on the molecular HHG in the two-color field by tuning the longer wavelength component from 1500nm to 1900nm.

2. Experiment and discussion

2.1 Experimental setup

The experimental setup is shown in Fig. 1(a)
Fig. 1 (a) Experimental setup. The two output beams are collinearly focused with variable time delay into a gas cell located in a high-vacuum interaction chamber. The generated high-order harmonics are detected by a home-made flat-field grating spectrometer equipped with a soft-x-ray CCD. (b) The tunable OPA spectra from 1500nm to 1900nm.
. A commercial Ti:sapphire femtosecond laser (Coherent Inc.) is used in this experiment, which can produce 8mJ/1kHz laser pulses at 800nm center wavelength with 40fs pulse duration. The output pulses are split into two beams. One beam (6mJ/1kHz) is used to pump a home-build infrared optical parametric amplifier (OPA) [12

12. C. Zhang, P. Wei, Y. Huang, Y. Leng, Y. Zheng, Z. Zeng, R. Li, and Z. Xu, “Tunable phase-stabilized infrared optical parametric amplifier for high-order harmonic generation,” Opt. Lett. 34(18), 2730–2732 (2009). [CrossRef] [PubMed]

], which can produce 1.0mJ/1kHz tunable pulses as the longer wavelength component. The central wavelength of the longer wavelength component can be tuned from 1500nm to 1900nm with ~45fs pulse duration each, and the full spectral width at half maximum intensity is about 120~180nm, which is shown in Fig. 1(b). The other beam (2mJ/1kHz) with 800nm central wavelength is used as the main driving pulses for generating the high harmonics. After a time delay line, the two color beams with the same parallel polarization are collimated and combined collinearly to be focused into a gas cell (the focal length of the lens is 150mm, and the length of the gas cell is ~2.5mm) located in a high-vacuum interaction chamber. The focal size of the tunable longer wavelength component is ~150µm and the Raleigh length is ~5mm, with the average intensity estimated to be ~0.6 × 1014 W/cm2 in the interaction region within the gas. The intensity of the tunable longer wavelength component is too weak to generate HHG in gas sell. The focal size of the other 800nm component is ~100µm and the Raleigh length is ~2.5mm, with the average intensity estimated to be ~1.2 × 1014 W/cm2 in the interaction region. The two focal spots are overlapped in space by adjusting the optical telescope. The stagnation pressure in the gas cell is controlled around 100 torr. The generated high-order harmonics are detected by a home-made flat-field grating spectrometer equipped with a soft-x-ray charge-coupled device camera (CCD, Princeton Instruments, SX 400), and Al foil with 500nm thickness is used in the spectrometer to stop the driving laser. The entire spectrum is imaged to the CCD array and averaged over multiple laser pulses.

2.2 Experimental results

We first measure the harmonic yields from CH4 molecules and Xe atoms as functions of the time delay between the two color pulses at the equal gas density driven by the same two-color field with the longer wavelength component tuned to 1700nm. The measured harmonic yields are shown in Figs. 2(a)
Fig. 2 Measured harmonic yields as functions of the time delay between the two color pulses from (a) CH4 molecules and (b) Xe atoms at the equal gas density driven by the same two-color field with the longer wavelength component tuned to 1700nm. (c)-(d) The harmonic spectra are the lineouts of (a)-(b) at the time delay of −100fs (red dotted lines), −50fs (green dashed lines) and 0fs (black solid lines).
, 2(b). The x-axis is the time delay between the two color pulses, and the positive value means that the tunable longer wavelength component is in prepropagation. The y-axis is the harmonic spectrum. The harmonic spectra shown in Figs. 2(c), 2(d) are the lineouts of Figs. 2(a), 2(b) at the time delay of −100fs (red dotted lines, beyond the superposition of the two color pulses), −50fs (green dashed lines, at the edge of the superposition) and 0fs (black solid lines, at the center of the superposition). We unambiguously observe that the harmonic yield from CH4 molecules is suppressed within the superposition of the two color pulses (from −50fs to + 50fs) while the harmonic yield from Xe atoms at the same laser condition is enhanced. Moreover, we measure the harmonic yield using only one laser beam respectively to ensure that the harmonic signal is mainly generated by the 800nm beam, and the laser intensity of the longer wavelength beam is too weak to generate the HHG in Xe or CH4 gas.

In order to further investigate the effect from the longer wavelength component, we also measure the harmonic yields from CH4 molecules and Xe atoms as functions of the laser intensity of the longer wavelength component at the time delay of 0fs within the superposition of the two color pulses shown in Fig. 3
Fig. 3 Measured harmonic yields as functions of the laser intensity of the 1700nm component at the time delay of 0fs from (a) CH4 molecules and (b) Xe atoms. The laser intensity of the 800nm component is kept constant.
. The laser intensity of the 800nm component is kept constant. It is shown that the HHG from CH4 molecules become weak and being suppressed with the increasing laser intensity of the longer wavelength component, while the HHG from Xe atoms is being enhanced and broadened with the increasing laser intensity of the longer wavelength component. They indicate that the 1700nm component can destructively suppress the HHG from CH4 molecules.

2.3 Discussion

The infrared absorption is attributed to the stretching and bending vibration of methane together with the interactions between the stretching and the bending modes. The maximum absorption peak of methane is the characteristic absorption peak of 3300nm, and the other higher absorption peaks are mainly focused on the wavelength from 1600nm~1800nm, such as the vibrational excitations of the 2v 1, 2v 3 and v 1 + v 3 mode [14

14. R. Lemus and A. Frank, “Algebraic approach to vibrational spectra of tetrahedral molecules: Application to methane,” J. Chem. Phys. 101(10), 8321 (1994). [CrossRef]

]. Therefore, there is a resonance between the infrared photon energy and one of the molecular vibrational energies when the CH4 molecules are driven by the OPA laser tunable from 1500nm to 1900nm. The infrared laser around the absorption resonance can excite the higher molecular vibrational state even dissociation [15

15. A. Talebpour, A. D. Bandrauk, J. Yang, and S. L. Chin, “Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse,” Chem. Phys. Lett. 313(5-6), 789–794 (1999). [CrossRef]

]. The excitation and the dissociation maybe together affect the HHG process in our experiment.

Macroscopically, we mainly compare the suppression of the molecular HHG with the enhancement of the atomic HHG at the same laser condition, and focus on the phenomenon that the longer wavelength component around the molecular infrared absorption can suppress the molecular HHG. The differences between CH4 molecules and Xe atoms are obvious in these experiments. For CH4 molecules, we have observed the suppression of the molecular HHG not only through the temporal overlap shown in Fig. 2 but also through the increasing intensity of the longer wavelength component shown in Fig. 3. For Xe atoms, the two-color field effect on the harmonic yield is normal and expected [16

16. Y. Zheng, Z. Zeng, X. Li, X. Chen, P. Liu, H. Xiong, H. Lu, S. Zhao, P. Wei, L. Zhang, Z. Wang, J. Liu, Y. Cheng, R. Li, and Z. Xu, “Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field,” Opt. Lett. 33(3), 234–236 (2008). [CrossRef] [PubMed]

20

20. I. Kim, H. Kim, C. Kim, J. Park, Y. Lee, K. Hong, and C. Nam, “Efficient high-order harmonic generation in a two-color laser field,” Appl. Phys. B 78(7-8), 859–861 (2004). [CrossRef]

], and the enhancement and broadening of the atomic HHG can be attributed to the two-color field effect and the increasing electric field intensity.

These observations are helpful for the future experiment on such molecular system and important for the theoretical study on this topic. According to the calculations of the nuclear motion effect on the molecular high-order harmonics [8

8. M. Lein, “Attosecond probing of vibrational dynamics with high-harmonic generation,” Phys. Rev. Lett. 94(5), 053004 (2005). [CrossRef] [PubMed]

,9

9. A. D. Bandrauk, S. Chelkowski, S. Kawai, and H. Lu, “Effect of nuclear motion on molecular high-order harmonics and on generation of attosecond pulses in intense laser pulses,” Phys. Rev. Lett. 101(15), 153901 (2008). [CrossRef] [PubMed]

], one knows that the molecular HHG should be sensitive to the nuclear motion induced by the laser field. Furthermore, the nuclear resonance induced by the infrared absorption is an intensive nuclear motion, and this intensive motion must play a key role on the molecular HHG. Our theoretical work in this field has made progress on the simplest molecule H2 which shows that the molecular HHG is intensively dependent on the driving laser wavelength [21

21. C. Liu, Z. Zeng, P. Wei, P. Liu, R. Li, and Z. Xu, “Driving-laser wavelength dependence of high-order harmonic generation in H2+ molecules,” Phys. Rev. A 81(3), 033426 (2010). [CrossRef]

]. However, due to the complexity of CH4 molecule, we give a qualitative explanation and attribute such phenomena to the nuclear resonance induced by the molecular infrared absorption. A more comprehensive theory, considering the wavelength effect and the molecular infrared absorption, is necessary for fully account for the underlying physics.

3. Summary

In conclusion, we experimentally compare the harmonic yield from CH4 molecules with that from Xe atoms in the two-color field with the same laser condition, and simultaneously observe the suppression of the HHG from CH4 molecules and the enhancement of the HHG from Xe atoms through by controlling the temporal overlap of the two color pulses or tuning the laser intensity of the longer wavelength component. The wavelength effect on the molecular HHG in the two-color field is also demonstrated by tuning the longer wavelength component from 1500nm to 1900nm. The results indicate that the longer wavelength component around the molecular infrared absorption can destructively suppress the molecular HHG.

Acknowledgement

This work is supported from the National Basic Research Program of China under Grant No.2006CB806000, the National Nature Science Foundation of China No.10734080, No.60908008, 60921004, the Knowledge Innovation Program of the Chinese Academy of Sciences under Grant No.KGCX-YW-417, the Fund of the State Key Laboratory of High Field Laser Physics and Shanghai Commission of Science and Technology under Grants No.09QA1406500.

References and links

1.

P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett. 71(13), 1994–1997 (1993). [CrossRef] [PubMed]

2.

G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, “Isolated single-cycle attosecond pulses,” Science 314(5798), 443–446 (2006). [CrossRef] [PubMed]

3.

T. Kanai, S. Minemoto, and H. Sakai, “Quantum interference during high-order harmonic generation from aligned molecules,” Nature 435(7041), 470–474 (2005). [CrossRef] [PubMed]

4.

C. Vozzi, F. Calegari, E. Benedetti, J. P. Caumes, G. Sansone, S. Stagira, M. Nisoli, R. Torres, E. Heesel, N. Kajumba, J. P. Marangos, C. Altucci, and R. Velotta, “Controlling two-center interference in molecular high harmonic generation,” Phys. Rev. Lett. 95(15), 153902 (2005). [CrossRef] [PubMed]

5.

J. Itatani, D. Zeidler, J. Levesque, M. Spanner, D. M. Villeneuve, and P. B. Corkum, “Controlling high harmonic generation with molecular wave packets,” Phys. Rev. Lett. 94(12), 123902 (2005). [CrossRef] [PubMed]

6.

J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pépin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, “Tomographic imaging of molecular orbitals,” Nature 432(7019), 867–871 (2004). [CrossRef] [PubMed]

7.

S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirila, M. Lein, J. W. G. Tisch, and J. P. Marangos, “Probing proton dynamics in molecules on an attosecond time scale,” Science 312(5772), 424–427 (2006). [CrossRef] [PubMed]

8.

M. Lein, “Attosecond probing of vibrational dynamics with high-harmonic generation,” Phys. Rev. Lett. 94(5), 053004 (2005). [CrossRef] [PubMed]

9.

A. D. Bandrauk, S. Chelkowski, S. Kawai, and H. Lu, “Effect of nuclear motion on molecular high-order harmonics and on generation of attosecond pulses in intense laser pulses,” Phys. Rev. Lett. 101(15), 153901 (2008). [CrossRef] [PubMed]

10.

C. Vozzi, F. Calegari, E. Benedetti, S. Gasilov, G. Sansone, G. Cerullo, M. Nisoli, S. De Silvestri, and S. Stagira, “Millijoule-level phase-stabilized few-optical-cycle infrared parametric source,” Opt. Lett. 32(20), 2957–2959 (2007). [CrossRef] [PubMed]

11.

C. Vozzi, G. Cirmi, C. Manzoni, E. Benedetti, F. Calegari, G. Sansone, S. Stagira, O. Svelto, S. De Silvestri, M. Nisoli, and G. Cerullo, “High-energy, few-optical-cycle pulses at 1.5 µm with passive carrier-envelope phase stabilization,” Opt. Express 14(21), 10109–10116 (2006). [CrossRef] [PubMed]

12.

C. Zhang, P. Wei, Y. Huang, Y. Leng, Y. Zheng, Z. Zeng, R. Li, and Z. Xu, “Tunable phase-stabilized infrared optical parametric amplifier for high-order harmonic generation,” Opt. Lett. 34(18), 2730–2732 (2009). [CrossRef] [PubMed]

13.

P. Wei, C. Zhang, C. Liu, Y. Huang, Y. Leng, P. Liu, Y. Zheng, Z. Zeng, R. Li, and Z. Xu, “Wavelength effect on atomic and molecular high harmonic generation driven by a tunable infrared parametric source,” Opt. Express 17(17), 15061–15067 (2009). [CrossRef] [PubMed]

14.

R. Lemus and A. Frank, “Algebraic approach to vibrational spectra of tetrahedral molecules: Application to methane,” J. Chem. Phys. 101(10), 8321 (1994). [CrossRef]

15.

A. Talebpour, A. D. Bandrauk, J. Yang, and S. L. Chin, “Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse,” Chem. Phys. Lett. 313(5-6), 789–794 (1999). [CrossRef]

16.

Y. Zheng, Z. Zeng, X. Li, X. Chen, P. Liu, H. Xiong, H. Lu, S. Zhao, P. Wei, L. Zhang, Z. Wang, J. Liu, Y. Cheng, R. Li, and Z. Xu, “Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field,” Opt. Lett. 33(3), 234–236 (2008). [CrossRef] [PubMed]

17.

F. Calegari, C. Vozzi, M. Negro, G. Sansone, F. Frassetto, L. Poletto, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver,” Opt. Lett. 34(20), 3125–3127 (2009). [CrossRef] [PubMed]

18.

C. Vozzi, F. Calegari, F. Frassetto, L. Poletto, G. Sansone, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Coherent continuum generation above 100eV driven by an ir parametric source in a two-color scheme,” Phys. Rev. A 79(3), 033842 (2009). [CrossRef]

19.

H. Merdji, T. Auguste, W. Boutu, J. P. Caumes, B. Carré, T. Pfeifer, A. Jullien, D. M. Neumark, and S. R. Leone, “Isolated attosecond pulses using a detuned second-harmonic field,” Opt. Lett. 32(21), 3134–3136 (2007). [CrossRef] [PubMed]

20.

I. Kim, H. Kim, C. Kim, J. Park, Y. Lee, K. Hong, and C. Nam, “Efficient high-order harmonic generation in a two-color laser field,” Appl. Phys. B 78(7-8), 859–861 (2004). [CrossRef]

21.

C. Liu, Z. Zeng, P. Wei, P. Liu, R. Li, and Z. Xu, “Driving-laser wavelength dependence of high-order harmonic generation in H2+ molecules,” Phys. Rev. A 81(3), 033426 (2010). [CrossRef]

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(020.2649) Atomic and molecular physics : Strong field laser physics

ToC Category:
Atomic and Molecular Physics

History
Original Manuscript: March 12, 2010
Revised Manuscript: April 29, 2010
Manuscript Accepted: May 15, 2010
Published: May 18, 2010

Citation
Pengfei Wei, Candong Liu, Chunmei Zhang, Yansui Huang, Yuxin Leng, Peng Liu, Yinghui Zheng, Zhinan Zeng, Ruxin Li, and Zhizhan Xu, "Molecular high harmonic generation in a two-color field," Opt. Express 18, 11664-11670 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-11-11664


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References

  1. P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett. 71(13), 1994–1997 (1993). [CrossRef] [PubMed]
  2. G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, “Isolated single-cycle attosecond pulses,” Science 314(5798), 443–446 (2006). [CrossRef] [PubMed]
  3. T. Kanai, S. Minemoto, and H. Sakai, “Quantum interference during high-order harmonic generation from aligned molecules,” Nature 435(7041), 470–474 (2005). [CrossRef] [PubMed]
  4. C. Vozzi, F. Calegari, E. Benedetti, J. P. Caumes, G. Sansone, S. Stagira, M. Nisoli, R. Torres, E. Heesel, N. Kajumba, J. P. Marangos, C. Altucci, and R. Velotta, “Controlling two-center interference in molecular high harmonic generation,” Phys. Rev. Lett. 95(15), 153902 (2005). [CrossRef] [PubMed]
  5. J. Itatani, D. Zeidler, J. Levesque, M. Spanner, D. M. Villeneuve, and P. B. Corkum, “Controlling high harmonic generation with molecular wave packets,” Phys. Rev. Lett. 94(12), 123902 (2005). [CrossRef] [PubMed]
  6. J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pépin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, “Tomographic imaging of molecular orbitals,” Nature 432(7019), 867–871 (2004). [CrossRef] [PubMed]
  7. S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirila, M. Lein, J. W. G. Tisch, and J. P. Marangos, “Probing proton dynamics in molecules on an attosecond time scale,” Science 312(5772), 424–427 (2006). [CrossRef] [PubMed]
  8. M. Lein, “Attosecond probing of vibrational dynamics with high-harmonic generation,” Phys. Rev. Lett. 94(5), 053004 (2005). [CrossRef] [PubMed]
  9. A. D. Bandrauk, S. Chelkowski, S. Kawai, and H. Lu, “Effect of nuclear motion on molecular high-order harmonics and on generation of attosecond pulses in intense laser pulses,” Phys. Rev. Lett. 101(15), 153901 (2008). [CrossRef] [PubMed]
  10. C. Vozzi, F. Calegari, E. Benedetti, S. Gasilov, G. Sansone, G. Cerullo, M. Nisoli, S. De Silvestri, and S. Stagira, “Millijoule-level phase-stabilized few-optical-cycle infrared parametric source,” Opt. Lett. 32(20), 2957–2959 (2007). [CrossRef] [PubMed]
  11. C. Vozzi, G. Cirmi, C. Manzoni, E. Benedetti, F. Calegari, G. Sansone, S. Stagira, O. Svelto, S. De Silvestri, M. Nisoli, and G. Cerullo, “High-energy, few-optical-cycle pulses at 1.5 µm with passive carrier-envelope phase stabilization,” Opt. Express 14(21), 10109–10116 (2006). [CrossRef] [PubMed]
  12. C. Zhang, P. Wei, Y. Huang, Y. Leng, Y. Zheng, Z. Zeng, R. Li, and Z. Xu, “Tunable phase-stabilized infrared optical parametric amplifier for high-order harmonic generation,” Opt. Lett. 34(18), 2730–2732 (2009). [CrossRef] [PubMed]
  13. P. Wei, C. Zhang, C. Liu, Y. Huang, Y. Leng, P. Liu, Y. Zheng, Z. Zeng, R. Li, and Z. Xu, “Wavelength effect on atomic and molecular high harmonic generation driven by a tunable infrared parametric source,” Opt. Express 17(17), 15061–15067 (2009). [CrossRef] [PubMed]
  14. R. Lemus and A. Frank, “Algebraic approach to vibrational spectra of tetrahedral molecules: Application to methane,” J. Chem. Phys. 101(10), 8321 (1994). [CrossRef]
  15. A. Talebpour, A. D. Bandrauk, J. Yang, and S. L. Chin, “Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse,” Chem. Phys. Lett. 313(5-6), 789–794 (1999). [CrossRef]
  16. Y. Zheng, Z. Zeng, X. Li, X. Chen, P. Liu, H. Xiong, H. Lu, S. Zhao, P. Wei, L. Zhang, Z. Wang, J. Liu, Y. Cheng, R. Li, and Z. Xu, “Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field,” Opt. Lett. 33(3), 234–236 (2008). [CrossRef] [PubMed]
  17. F. Calegari, C. Vozzi, M. Negro, G. Sansone, F. Frassetto, L. Poletto, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver,” Opt. Lett. 34(20), 3125–3127 (2009). [CrossRef] [PubMed]
  18. C. Vozzi, F. Calegari, F. Frassetto, L. Poletto, G. Sansone, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Coherent continuum generation above 100eV driven by an ir parametric source in a two-color scheme,” Phys. Rev. A 79(3), 033842 (2009). [CrossRef]
  19. H. Merdji, T. Auguste, W. Boutu, J. P. Caumes, B. Carré, T. Pfeifer, A. Jullien, D. M. Neumark, and S. R. Leone, “Isolated attosecond pulses using a detuned second-harmonic field,” Opt. Lett. 32(21), 3134–3136 (2007). [CrossRef] [PubMed]
  20. I. Kim, H. Kim, C. Kim, J. Park, Y. Lee, K. Hong, and C. Nam, “Efficient high-order harmonic generation in a two-color laser field,” Appl. Phys. B 78(7-8), 859–861 (2004). [CrossRef]
  21. C. Liu, Z. Zeng, P. Wei, P. Liu, R. Li, and Z. Xu, “Driving-laser wavelength dependence of high-order harmonic generation in H2+ molecules,” Phys. Rev. A 81(3), 033426 (2010). [CrossRef]

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