Let’s consider the scattering of attosecond laser pulses on single-stranded DNA macromolecules shown in
Figure 1. In this work, we study single-stranded DNA macromolecules, since we are interested in comparing and contributing to the scattering spectra of an undamaged DNA chain (
Figure 1(a)) with a damaged one ((
Figure 1(b) and ( c)). Obviously, if calculations on single-stranded DNA macromolecules differ from the same calculations using the previous theory, then the same will happen for double-stranded structures. We will also carry out calculations on the same structures in the case of using the previous XRD theory, i.e. where the pulse duration is considered infinitely long. Let us consider in more detail the three structures presented above, on which the USP falls, see
Figure 2,
Figure 3 and
Figure 4.
Since we are considering attosecond pulses, we can use the sudden perturbation approximation to find the scattering spectra of ultrashort pulses. In this approximation, the theory of ultrashort pulse scattering was developed in the works [
16,
17,
19,
25,
26]. In this approximation, it is assumed that the duration of an ultrashort pulse
is many times shorter than the characteristic atomic time
, i.e.
. Let us recall that for sufficiently light atoms
. Also, this theory is also suitable for longer pulses [
27], including femtosecond ones. If we consider the USP to be spatially inhomogeneous, i.e. electromagnetic field strength
, where
is the field amplitude,
is an arbitrary function that determines the shape of the USP,
c is the speed of light (in a.e.
), then the scattering spectra (scattering energy and unit solid angle) of a Gaussian pulse
(
,
this is the Fourier transform of
,
is the carrier frequency of the pulse) can be represented as [
16,
17,
25,
26]
where
is the number of atoms of
i type;
is the number of electrons in the atom
i type;
is a radius vector specifying the position of an atom
i of type with number
; the summation is carried out over all atoms
and over all types of atoms
;
has the meaning of recoil momentum when light with frequency
is scattered by a bound electron and
;
is the form factor of the
i atom of the variety with electron density
. The electron density of the atoms
variety
i will be chosen in the independent atom model [
28]. In this case we get
, where
are constant coefficients defined in [
28]. It should be added that Eq.(
2) was obtained in the case of multi-cycle pulses, i.e.
. This case is precisely realized on well-known ultrashort pulse X-ray sources, for example, Free Electron Lasers (XFELs).
Let us add that if in Eq. (
2) we increase
, we get the known Eq. (
1). Thus, Eq. (
2) contains characteristics responsible for the duration of USP
. Indeed, if
, then the parameter
, and Eq. (
2) becomes proportional to
(Fermi’s golden rule), and therefore coincides with Eq. (
1). In this case, it is especially interesting to understand the physical meaning of the
momentum, since it is precisely this that is responsible for the main difference between Eq.(
2) and Eq.(
1). Its physical meaning is quite easy to understand if we consider that the incident USP is non-monochromatic, i.e. this pulse has a frequency dispersion proportional to
. This means that during USP scattering, a recoil momentum lying in the interval
is transferred to atomic electrons. From Eq.(
2), namely the parameter
it is clear that the momentum
can be transferred not to all electrons in the system under consideration, but only to those where the distance between electrons is of the order of
. In other words, due to frequency dispersion, it is not a plane wave of infinite extent that falls, but a wave packet of limited size
; accordingly, only those electrons that are located inside the region of space
are scattered together. As a result of this analysis, we can say that if we consider a structure whose asymmetrical part dimensions are comparable to or greater than
, then the pulse duration will have a significant contribution to the scattering spectra, i.e. there will be a big difference between Eq.(
2) and Eq.(
1). If we choose attosecond pulses with
and even an order of magnitude more than this, then such systems can be various macromolecules, including DNA, RNA, various proteins, etc. Although the exact quantitative contribution can only be determined as a result of direct calculations.