kleine_2022#

C. Kleine, M.-O. Winghart, Z.-Y. Zhang, M. Richter, M. Ekimova, S. Eckert, M. J. J. Vrakking, E. T. J. Nibbering, A. Rouzee and E. R. Grant, “Electronic State Population Dynamics upon Ultrafast Strong Field Ionization and Fragmentation of Molecular Nitrogen”, Phys. Rev. Lett. 129, 123002 (2022)

Source record · Catalogue · Review register

DOI: 10.1103/PhysRevLett.129.123002

Source kind: measurement

Recorded technique: Femtosecond soft-x-ray transient absorption at the nitrogen K edge (p. 2): N2 in a gas jet is strong-field ionised by an 800 nm pump pulse ‘with a peak intensity of 4.5 x 10^14 W cm^-2’ and probed by high harmonics generated in helium (200-450 eV), dispersed by a reflective-zone-plate spectrometer onto an x-ray CCD. Pump-probe delays -100 to +300 fs, and scans to 80 ps. The N2+ X, A and B populations come from Gaussian fits of the core-to-valence lines (394.4 eV, 1s -> 3sigma_g, X; 392.9 eV, 1s -> 1pi_u, A; 391.2 eV, 1s -> 2sigma_u, B; p. 2) weighted by relative oscillator strengths 0.8, 1.0 and 0.15 taken from the x-ray emission spectra of Glans et al. 1996 (their ref. 42), Eq. (1), p. 4. The N and N+ appearance from 1s -> 2p lines near 396, 397 and 399 eV (Fig. 4), with time constants by singular value decomposition (p. 4). The experimental set-up is described in the Supplemental Material (their ref. 28), which is not in the library: THE PUMP PULSE DURATION AND POLARISATION ARE NOT STATED IN THE LETTER.

Materials in structured entries: N2, N2+

Dataset quantities: dissociation_time, ionic_state_population

Contents: 5 datasets; 0 formulas.

Curation notes#

Journal article 123002 = pdf pages 1-7. Identity checked against pdf p. 1 (received 21 April 2022, published 14 September 2022); open access (CC BY 4.0). The Supplemental Material is not held. Curated 2026-10-09 (TC-958).

THE PULSE. 800 nm; ‘a peak intensity of 4.5 x 10^14 W cm^-2’ (p. 2), repeated as ‘~4.5 x 10^14’ for U_p = 29 eV (p. 5); no intensity calibration is described in the Letter. The duration is not stated in the Letter (only in the Supplemental Material, not held): pulse_fs is ‘not stated’. The polarisation is not stated; the alignment model of Fig. 3(a) (an ADK focal-volume ionisation with <cos^2 theta> to ‘the laser polarization axis’, p. 3) implies a linearly polarised pump.

THE STATE POPULATIONS (p. 4, Eq. 1; Fig. 3(b)): rho_ij = (A_ij w_ij / f_j) / N with N the sum over j of the same ratio averaged over the delays 160-300 fs, i.e. fractions of the N2+ ions in X, A and B together, after the pulse. ‘Surprisingly, after SFI the populations of the X 2Sigma_g+ and B 2Sigma_u+ states of 0.47 +- 0.02 and 0.46 +- 0.07 respectively, are almost identical while the population of the A 2Pi_u state remains low (0.07 +- 0.01)’ (p. 4). The confidence level is not stated. The three sum to 1.00. They depend wholly on the relative oscillator strengths 0.8 : 1.0 : 0.15 (X : A : B) of their ref. 42: B’s line is weighted up by 1/0.15. The vibrational distribution within each state is not resolved. An AC Stark shift of the fitted line centres is seen during the pulse (p. 4).

THE IONISED FRACTION (not a dataset; no quantity holds it). The bleach of the N2 1s -> 1pi_g resonance near 401 eV after the pulse is ‘Delta A = 150 mOD, ~61 +- 4 %’ of the absorbance (p. 2): a depletion of neutral ground-state N2 along the probe path, not a focal-volume ionisation probability. The rate estimate of p. 5 assumes ‘that 60% of molecules are ionized in the focal volume’. ionization_yield would need a numeric pulse_fs and an intensity dependence, neither of which is printed.

THE N + N+ APPEARANCE (p. 4-5; Fig. 4). Beyond about 1 ps the N2+ absorption decays and N and N+ lines appear; ‘We deduce a time constant of tau = 19.8 +- 1.5 ps for an N2 backing pressure of 4 bar [Fig. 4(c)]’, the rise of N and N+ and the ‘concomitant decay’ of N2+, ‘this time constant strongly depends on the backing pressure ... the time constant decreases when the pressure increases’ (Fig. 4(d), a fit ‘using an inverse function 1/p’). Assigned to collisions in the plasma: ‘Dissociative electron-ion recombination is expected to be of minor importance ... we expect that the density of energetic electrons is large enough to lead to significant collisional dynamics in the plasma, resulting in dissociative excitation of [N2+] ions’ (p. 5). The estimate of Eq. (2), k = n_e int sqrt(2E/m_e) rho(E) sigma(E) dE with the dissociative-excitation cross sections of their refs. 49-50, a plateau electron distribution to 2 U_p, the gas density of the Supplemental Material and 60 % ionisation, gives ‘a rise time tau = 1/(k ln(2)) of ~28 ps’ (p. 5). The definition 1/(k ln 2) is as printed (a half-life would be ln 2 / k); kept as printed. The scope note of the library copy: the 19.8 ps is not a prompt optical dissociation time and the jet pressure is not uniform. A small N+ shoulder at 399.7 eV within 300 fs ‘suggests the ultrafast formation of N+ by dissociative ionization’ (p. 3); no number.

QUOTED, NOT TAKEN: N2’s ionisation potentials for HOMO, HOMO-1, HOMO-2, 15.58, 17.07 and 18.75 eV (p. 1, no source); U_p = 29 eV at 4.5 x 10^14 W cm^-2 (p. 5); the alignment-induced minimum of the N2+ 1s -> 1pi_g signal near 50 fs (p. 3).

FIGURES (not digitised; vector drawings on pdf pp. 3-4): Fig. 3(b), the X, A and B populations against delay to 300 fs; Fig. 4(c), the N and N+ signals to 80 ps with the single-exponential fits; Fig. 4(d), the N2+ dissociation time against backing pressure with the 1/p fit (the only pressure dependence; worth reading).

Recorded bibliographic identity status: VALIDATED. This concerns identification of the source; it does not establish the scientific correctness of its contents.