Частоты в дискретном преобразовании Фурье

import matplotlib.pyplot as plt
import numpy as np
 
 
def autocorr_func(t, gamma,  w0, S, w):
    arg = w0 * t * 1j - gamma * t
    for i in range(len(S)):
        arg += S[i] * (np.exp(1j * w[i] * t) - 1)
    return np.exp(arg, dtype=complex)
 
 
hrf_ = np.array(list(map(float, open("hrfactor.txt", 'r').readlines())))            # read "hrfactor.txt
freq_ = np.array(list(map(float, open("freq.txt", 'r').readlines()))) * 0.45216e-5  # read "freq.txt" [cm-1] to [a.u.]
w_eg = 29255.96 * 0.45216e-5 # [cm-1] to [a.u.]
 
 
fig1, (ax1, ax2) = plt.subplots(
    nrows=2, ncols=1,
    figsize=(12, 10)
)
 
delta_t = 4.134137333               # [a.u.] = 0.1 [fs]
N = int(1e5)                        # number of points
t = np.linspace(0, delta_t * N, N)  # time array [a.u.]
 
G_ab = autocorr_func(t, gamma=0.001, w0=w_eg, S=hrf_, w=freq_)  # in [a.u.]
 
yf = np.real(np.fft.fft(G_ab))[: N // 2]
xf = np.linspace(0.0, 1/(2.0 * delta_t), N // 2) / 0.45216e-5  # [a.u.] to [cm^-1]
 
ax1.plot(t, np.abs(G_ab))
ax1.set_ylabel("ABS(G_ab(t)")
ax1.set_xlabel("Time, a.u.")
 
ax2.plot(xf, yf)
ax2.set_ylabel("I_ab(w)")
ax2.set_xlabel("Wavenumber, cm^-1")
 
plt.show()