Display JavaScript Alert MessageBox after clicking Logout Button and then redirect in ASP.Net

this is my code

protected void btnLogout_Click(object sender, EventArgs e)     {

Session.Clear();         Session.RemoveAll();         Session.Abandon();                 ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Thanking You Visitng Trinity Consultancy Services.')</script>");          Response.Redirect("~/Login.aspx", true);

}

on logout button click its directly going to login.aspx its  not showing the message

i want it should show the message and then redirect.